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Representing temporal discourse markers for generation purposes Brigitte Grote

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Abstract

Discourse markers are an important means to signal the kind of coherence relation holding between adjacent text spans. Research on generating discourse markers has been mainly concerned with causal markers, whereas temporal markers have not received much attention. In this paper, we identify semantic, pragmatic and syntactic features that axe required to support a motivated choice of German temporal subordinating conjunctions and prepositions during text production. Information on individual markers is assembled in a discourse marker lexicon, which is used as a declaxative resource at the sentence planning stage. We illustrate how this resource can be used to produce alternative verbalizations of the temporal relationship holding between two events.
Representing temporal discourse markers for generation purposes
Brigitte Grote
Otto-von-Guericke Universit~it Magdeburg
Institut fiir Wissens- und Sprachverarbeitung
P.O. Box 4120, 39016 Magdeburg, Germany
emaih grote~iws, cs. uni-magdeburg, de
Abstract
Discourse markers are an important means to signal
the kind of coherence relation holding between adja-
cent text spans. Research on generating discourse
markers has been mainly concerned with causal
markers, whereas temporal markers have not received
much attention. In this paper, we identify semantic,
pragmatic and syntactic features that are required
to support a motivated choice of German temporal
subordinating conjunctions and prepositions during
text production. Information on individual markers
is assembled in a discourse marker lexicon, which is
used as a declarative resource at the sentence plan-
ning stage. We illustrate how this resource can be
used to produce alternative verbalizations of the tem-
poral relationship holding between two events.
1 Motivation
In text, discourse markers signal the kind of coher-
ence relation holding between adjacent text spans.
For any but the most trivial applications of language
generation, motivated marker choice is an important
task. Whereas several studies have been concerned
with causal markers and their interactions with other
linguistic means, for instance, Vander Linden and
Martin (1995), Rhsner and Stede (1992), Delin et al.
(1996), temporal markers as signals of the temporal
relation holding between two events have not received
much attention, with the exception being Dorr and
Gaasterland (1995). However, quite often, it is only
by means of explicit temporal markers that the cor-
rect interpretation of a text can be ensured, as the
following examples illustrate:
(1) Der
Abstand ist nachzumessen, nachdem das Band
mindestens einen Umlauf ausge~hrt
hat. (Check
the distance after the belt has completed at least
one round.)
(2)
W~ihrend Sie den Toaster betreiben, die
Brotschlitze nicht abdecken.
(While operating the
toaster, do not cover the bread slots.)
In both cases, the order of events as recounted in
the text does not correspond to their order of oc-
curence: In example (1),
nachdem
(after) marks the
event denoted in the second clause as temporally an-
terior to the one denoted in the first clause. In ex-
ample (2),
wiihrend
(while) marks both events as
cotemporal. Without a temporal marker, the order
of events would not be obvious.
Selecting an appropriate discourse marker for a
given temporal relation is by no means a straightfor-
ward matter, though. First, one and the same tem-
poral relation is verbalized differently depending on
the syntactic and lexical properties of the clauses it
conjoins. In German, the language we consider, such
properties are, among others, tense, aspect, and syn-
tactic structure. Interdependencies occur when two
temporal events are realized in the same sentence; we
will therefore restrict the study of German temporal
markers to subordinating conjunctions and preposi-
tions. Second, temporal discourse markers can even
overwrite the temporal relations indicated by other
linguistic means like tense and aspect, as noted by re-
searchers who work in the analysis of temporal mark-
ers (e.g. Hitzeman et al. 1995).
In a nutshell, when aiming at selecting an ap-
propriate temporal marker in text generation one
needs a representation of temporal markers that en-
ables marker choice and other sentence level decisions
(such as tense and aspect selection) to mutually con-
strain each other. In this paper, we take up the idea
suggested in Grote and Stede (1998) of a discourse
marker lexicon as a declarative resource at the sen-
tence planning stage. We demonstrate how such a
lexicon for temporal markers can be employed in text
generation to produce different verbalizations of the
same underlying temporal organization depending on
other generation decisions.
The paper is organized as follows: Section 2 re-
views related work on generating temporal mark-
ers. Section 3 describes the major semantic, prag-
matic and syntactic properties of German temporal
prepositions and subordinating conjunctions. Sec-
tion 4 presents the generation perspective: It briefly
discusses the shape of the discourse marker lexi-
con, introduces the features used in the lexicon, and
presents sample lexicon entries and their application
in the generation process.
22
2 Related work
Work on discourse marker generation in general has
focussed on marker selection, mainly for causal re-
lations (Elhadad and McKeown 1990; Vander Lin-
den and Martin 1995), and on the realization of
RST's subject-matter relations (Rhsner and Stede
1992; Delin et al. 1996). As for temporal mark-
ers, Dorr and Gaasterland (1995) examine the gen-
eration of English temporal subordinating conjunc-
tions. Gagnon and Lapalme (1993), on the other
hand, describe the generation of French temporal ad-
verbs based on a DRT representation of the discourse.
While Gagnon and Lapalme (1993) only briefly
address conjunctions and prepositions, Dorr and
Gaasterland (1995) present a detailed study of tem-
poral connectives, but they consider English mark-
ers only. The only account on automatically produc-
ing German temporal expressions that we know of is
Ehrich (1987); however, she discusses the interaction
of tense and aspect in simple sentences only.
Most studies that deal with discourse markers re-
gard their production as a mere consequence of other
sentence level decisions such as aggregation, lexical-
ization, syntactic structuring, and--in the case of
temporal markers--as determined by tense and as-
pectual choices. We believe, however, that one needs
a more flexible control to increase the expressiveness
of generation systems. Although there have been
quite a few studies on individual aspects of sentence
planning, little attention has been paid to the in-
teraction between the various tasks--exceptions are
Rambow and Korelsky (1992) and Wanner and Hovy
(1996)--and in particular to the role of marker choice
in the overall sentence planning process.
There exists a large body of research in NLU on
analysing the temporal structure of texts, including
the role of temporal markers, though again restricted
to English (Moens and Steedman 1988; Lascarides
and Oberlander 1993; Hitzeman et al. 1995). We turn
to these studies when it comes to identifying the in-
formation that needs to be assembled for representing
temporal markers.
3 Linguistic perspective:
Describing temporal markers
Selecting an appropriate German temporal marker
given two events in a temporal relationship requires
detailed knowledge of the semantic, pragmatic and
syntactic properties that characterize temporal mark-
ers. This section introduces the major properties and
explores
the correlations between temporal markers
and other linguistic means that indicate temporal or-
ganization. We base our account on two sources:
descriptive linguistic studies, mainly by Helbig and
Buscha (1991), B/iuerle (1995), Buscha (1989) and
Steube (1980); and our analysis of temporal marker
usage in the German LIMAS corpus (Glas 1975).
3.1 The 'meaning' of German temporal
markers
Temporal subordinating conjunctions and temporal
prepositions conjoin two events where the event in the
subordinate clause (or the PP) provides the tempo-
ral framework for interpreting the event in the main
clause: Bevor Sie den Toaster reinigen, den Net-
zstecker ziehen. (Before you clean the toaster, un-
plug the device.) and the corresponding 'shorthand'
form l br dem Reinigen des Toasters den Netzstecker
ziehen (Unplug before cleaning the toaster).
Semantic properties German grammars such as
Helbig and Buscha (1991) list about 20 temporal
subordinating conjunctions and 20 temporal prepo-
sitions. Their semantics is usually described by the
kind of temporal relation they establish between two
events, see for instance, Steube (1980) and Helbig and
Buscha (1991): The event in the main clause can ei-
ther overlap with (simultaneity), succeed (anteri-
ority), or precede (posteriority) the event depicted
in the subordinate clause or the prepositional phrase.
In table 1 we provide a synthesis of the classifica-
tions of the most frequent German temporal mark-
ers by Helbig and Buscha (1991), Buscha (1989) and
B/iuerle (1995). The markers listed in the table re-
flect the scope of the marker study in this paper.
Two aspects are especially prominent: First, each
of the three temporal relations can be realized by a
number of temporal markers. Alternatives within a
class differ in that they realize some additional mean-
ing aspect. Consider the markers of simultaneity:
Solange, for instance, conveys the idea of a strict si-
multaneity where two events have the same start and
end time, and is more specific than w~hrend; sooft,
to give another example, highlights the concurrence
of two events.
Second, table 1 shows that some markers are am-
biguous: Als and wenn occur in all three classes, seit-
dem, sobald and sooft in two. Apparently, neither
of them has any special temporal implicature on its
own; instead, these markers depend on syntactic and
lexical contexts to receive an umambiguous temporal
meaning. We will return to this issue in section 3.2.
Pragmatic properties The choice Of a particular
marker to express a temporal relation between two
events interacts with the focus structure as in:
(3) (a) Bevor ihr Mann das Haus verlie[J, ging sie zur
Arbeit. (Before her husband left the house, she
went to work.)
(b) Nachdem sie zur Arbeit gegangen war, verliefl
ihr ivIann das Haus. (After she had gone to work,
her husband left the house.)
Alternatives (3a) and (3b) both express that the
event of 'going to work' precedes the event of 'leav-
ing the house'. They differ in that they focus on
23
temporal relation temporal markers
simultaneity subc: als
(as), indes(sen) (meanwhile), seitdem (since), sobald (as
soon as), solange
(as long as), sooft (whenever),
sowie (as soon as),
w~hrend (while),
wenn (when)
prep:
an
(at), auf (on), bei (during),
binnen
(within), durch (for), in (in), iiber (over),
w~hrend (during)
anteriority subc: a./s
(when), kaum dab (no sooner),
nachdem (after),
seit(dem) (since),
sobald (as
soon as),
sooft (whenever), sowie (as soon as), wenn (when)
prep: ab (from), nach (after),
seit
(since)
posteriority subc:
als (when), bevor (before), his (until), ehe (before),
wenn
(when)
prep: his (until), vor (before)
Table 1:
German temporal subordinating conjunctions (subc) and prepositions (prep) classified by temporal
relations. Note that
the
corresponding English markers
are
only approximate translations.
different aspects of the situation: In (3a) the ear-
lier event is in the centre of attention, in (3b) the
later one (assuming that the matrix sentence is more
prominent). This phenomenon interacts with other
discourse phenomena, for instance, given and new
information, and--when placed in a larger discourse
context--with presuppositions and their accommo-
dation (Lascarides and Oberlander 1993). However,
the treatment of the discourse behaviour of temporal
markers is beyond the scope of this paper.
Pragmatic issues further concern style. Regard-
ing temporal markers, stylistic features are of minor
importance: We only observe variation between ar-
chaic and neutral (da vs. als), and formal and neutral
(kaum dab vs.
sobald)
markers.
3.2 Syntactic and lexical constraints
When expressing several events in the same sentence,
marker choice interacts with other linguistic means:
Temporal markers impose particular constraints on
the syntactic and lexical contexts they can occur in.
Conversely, these contexts can influence the meaning
of markers. 1
Markers and Aktionsart/aspect Aspect is tra-
ditionally taken to have two components, the non-
inherent grammatical features, and the inherent lex-
ical features. Inherent features characterize facets of
the situation denoted by a verb, for instance, whether
it is an event or a state. We will label these fea-
tures
Aktionsart
to avoid confusion. According to
Bussmann (1990), the major Aktionsarten in German
are stative
(wissen/to know)
and dynamic. For the
latter, the basic dichotomy is that between durative
(schlafen/to
sleep) and non-durative verbs, which are
subdivided into iterative
(flattern/to flap),
semelfac-
tive
(klopfen/to knock),
resultative
(verbrennen/to
burn up)
and causative verbs
(trii.aken/to
water).
1Traditional grammars, which the present account is based
on, usually list aspect, Aktionsart and tense as constraining
parameters on marker choice. However, there is no consensus
on the role of these parameters; B~.uerle (1995) provides a good
overview of the range of positions.
Two kinds of interdependencies are generally ac-
knowledged, see Ehrich (1987), Buscha (1989) and
B~iuerle (1995). First, temporal markers are sensi-
tive to the Aktionsart of a verb. Consider
w~ihrend
and als which can both express simultaneity:
(4) (a)
Als das Kabel schmolz / riB,
war
ich nicht im
Raum.
(When the cable melted / tore, I wasn't in
the room.)
(b)
W~hrend das
Kabel
schmolz / *riB, war ich
nicht im Raum.
(While the cable melted / *tore, I
wasn't in the room.)
Wghrend
expects a durative verb in the subordi-
nate clause, hence it can occur with
schmelzen/to
melt
but not with
reit3en/to tear. Als,
in contrast,
can be used with durative and resultative verbs, as
(4a) illustrates. Second, temporal markers may even
shift the Aktionsart of a verb, for instance from a
semelfactive reading to an iterative one as in:
(5) (a)
Wenn
es an
der Tfir klopft, schreit das Baby.
(When someone knocks at the door, the baby cries.)
(b)
Wiihrend es an der Tfir klopft, schreit das
Baby.
(While someone knocks at the door, the
baby cries.)
Grammatical aspect reflects the individual per-
spective a speaker adopts with respect to an event,
such as perfective (temporally closed) or imperfec-
tire. In German, this distinction is grammatically
realized by choosing a perfective or simple tense, e
Aktionsart and aspect closely interact, consider ex-
ample (6) where the anterior reading (6b) is due to
the use of a perfective tense with a non-durative verb
in the subordinate clause, which indicates that the
activity has been concluded:
(6) (a)
Seitdem ich ihn kenne, ist
er
Nichtraucher.
(Since I know him, he is a non-smoker.)
(b)
Seitdem seine I~rau gestorben ist, sehe ich ihn
nur selten.
(Since his wife has died, I only rarely
see him.)
Sin contrast to English, and especially to slavic languages,
German has no elaborate aspect system: Distinctions like pro-
gressive and simple cannot be signalled by morphological fea-
tures of the verb, but require a separate temporal adverb: He
is reading
vs.
Sie 1lest gerade
(She reads right now).
24
Here, verb properties determine the reading of the
temporal marker. Our study of temporal marker oc-
currences in the LIMAS corpus suggests that mark-
ers belonging to the simultaneity class typically real-
ize imperfective aspect, whereas temporal connecting
words that signal anteriority correlate with a perfec-
tive aspect in the subordinate clause.
Markers and verbal tense Some markers can
only be used with particular tenses, for instance, a/s
in its simultaneous reading cannot occur with present
tense, whereas
wenn as
signal of simultaneity corre-
lates with present and past tense:
(7) (a)
Als
er
in Dresden war (*ist), suchte (,sucht) er
seine 1~reundin
au£
(b)
Wenn
er
in Dresden ist / war, sucht / suchte er
seine Freundin
au£
However, tempus sensitivity of temporal markers is
not a matter of the grammatical tense form (such as
simple past, present perfect, etc.) but relates to the
temporal structure of the individual events, and to
how their temporal structures are related. Assum-
ing the Reichenbachian threefold distinction between
Event Time (E), Reference Time (R), and Speaking
Time (S) (the Basic Tense Structure, BTS, (Reichen-
bach 1947)), we observe that the constraints imposed
by a marker on verb tense concern the underlying re-
lation between E and S of both clauses: Selecting
either a/s or
wenn
to express simultaneous events in
the main clause (era) and in the subordinate clause
(es) depends on whether the event times precede S
(E(em),E(es)_S) or concur with S (E(em),E(es),S). 3
The grammatical tense results from combining the
BTS of both clauses and their aspectual features.
Markers and syntactic structure The most
straightforward correlation is that between syntac-
tic structure and marker choice: If two events are
expressed by a hypotactic structure, a subordinating
conjunction is required. When a deverbal realization
of an event is possible (e.g.
treffen/das
Treffen; to
meet/the meeting), a clause with an adverbial (tem-
poral) prepositional phrase is realized.
Markers and temporal quantifiers With some
markers, the temporal relation denoted by the marker
can be quantified by a temporal adverb as in
kurz
bevor
(shortly before) or
einige Stunden nachdem
(several hours after); others cannot be quantified:
einige Stunden sobald
(.several hours as soon as).
4 Generation perspective:
Representing temporal markers
A representation of temporal markers suitable for
generation purposes has to accommodate the follow-
ing demands: First, it has to describe the semantic
SThe comma stands for 'is cotemporal', the underscore for
'precedes'.
and pragmatic features of markers in a manner that
supports a motivated choice between markers which
can realize the same temporal relation. Second, it
has to account for the constraints temporal mark-
ers impose on their syntactic and lexical contexts,
thereby enabling interactions between marker choice
and other sentence planning decisions where the order
of decision-making is not fixed. In Grote and Stede
(1998) we argue that such a flexible control is best
realized by introducing independent modules for the
different sentence planning tasks, such as proposed
by Wanner and Hovy (1996), and that these modules
should rely on declarative representations as much as
possible. Therefore, we propose a discourse marker
lexicon, i.e. an independent lexical resource that as-
sembles specifically the information associated with
discourse markers.
Traditional lexicology and grammars describe lexi-
cal entries along three features: semantic, pragmatic
and syntactic dimensions (see section 3). From the
production perspective, these features are to be clas-
sifted with respect to when and where they come into
play in the generation process; this amounts to a pro-
cedural view on the information coded in the lexicon.
Following Grote and Stede (1998) we assume three
categories in the marker lexion:
Applicability conditions: The necessary con-
ditions that need to be present in the input rep-
resentation for the marker to be a candidate.
Chiefly, this is the semantic/discourse relation
to be expressed, and also (if applicable) features
pertaining to presuppositions and intentions.
Combinability conditions: The constraints
that the marker imposes on its neighbouring lin-
guistic constituents (the 'syntagmatic' dimen-
sion). These are syntactic constraints on subcat-
egorization and semantic type constraints, which
interact with other realization decisions in sen-
tence planning.
Distinguishing features: If preferential choice
dimensions, such as style, brevity, etc., are at-
tended to in the system, then these features serve
to distinguish markers that are otherwise equiv-
alent (the 'paradigmatic' dimension).
In the remainder of this section we describe lexicon
entries for temporal markers along these lines.
4.1 Applicability conditions
Semantic conditions The semantic classes intro-
duced in section 3.1 (simultaneity, anteriority and
posteriority) turned out to be too coarse for genera-
tion purposes. Instead, one needs a more fine-grained
representation of the semantics of temporal markers
to support an informed choice among markers within
the broad classes.
Allen's temporal interval relationships provide an
adequate framework (Allen 1984), as already sug-
25
gested by Dorr and Gaasterland (1995). Allen in-
troduces seven basic temporal interval relationships,
namely
equals(=), after(>), during(d), overlaps(o),
meets(m), starts(s), finishes(f)--and
their inverses
<,di, oi, mi, si,fi--that
may exist between two events
em
and es. For instance,
overlaps(em,e~) as
in (4b)
implies that there is an intersection between the time
at which em occurs and the time at which
es
occurs,
but that neither event is a subset of the other.
Each temporal relation corresponds to one or sev-
eral German temporal markers, for instance,
overlaps
may be expressed by the entire range of simultane-
ity markers given in table 1, except for solange and
kaum daB. Conversely, the majority of the tempo-
ral markers can realize several temporal interval re-
lations. Take the connective
nachdem as
in example
(1), which can have the following meanings,
after(ern, es)
A
meets-i(em, es)
or
w~ihrend as
in example (4b),
equals(ern, es) A during(era, es) A starts(era, es)
A finishes(era, es) A overlaps-i(em, es)
whereas solange has only one reading:
equals(era, es ).
This adequately captures the semantic difference
between
w&hrend
and
solange.
In the lexicon, the ap-
plicability conditions of a particular temporal marker
are now described by listing the temporal interval re-
lations it can realize.
Pragmatic
conditions In section 3.1 we briefly
discussed pragmatic features of temporal markers.
For the time being, the lexicon supports the features
style, with the values neutral, brief, formal,
archaic, and intention. Its value evaluative in-
dicates the speaker's (negative) attitude towards the
kind of temporal relation holding between two events
(Steube 1980; Buscha 1989).
4.2 Combinability conditions
Combinability conditions appear as constraints in the
lexicon entries of individual markers. In the present
lexicon, constraints are described using the following
features:
Aktionsart The Aktionsart plays a central role
during the lexicalization of events: Candidate verbs
are, among others, selected due to their Aktion-
sart. Aktionsart features are usually stored in the
lexicon entries of verbs, and are thus available to
sentence planning. To represent these constraints,
we turn to Bussmann (1990) for the major Aktion-
sarten in German (see also section 3.2). 4 At present,
the lexicon supports a subset of Bussmann's Aktion-
sarten, namely stative, durative, iterative,
semelfactive, causative and resultative.
4There is no
generally accepted and
well-defined set of Ak-
tionsart features; we
opted for
Bussmann (1990) because these
features are supported by
the lexicalization component we in-
tend to use (Stede 1996).
Aspect Grammatical aspect is encoded using the
feature values perfective and imperfective.
Tense
We argued above that marker choice relates
to the underlying temporal structure---as expressed
in terms of the Reichenbachian threefold description
of time-and not to a particular grammaticai tense
(see also Ehrich (1987)). Temporal constraints in the
marker lexicon will thus be described using the BTS
notation, and defining the legal linear orderings of
E, R and S of the related events. For instance,
als
in its simultaneous meaning imposes the constraint
E(e~),E(e~)_S,
which can be realized by all gram-
matical tenses that meet this constraint.
Mapping this representation into grammatical
tense requires knowledge on how to map pairs of Ba-
sic Tense Structures to the tense structure of complex
German sentences, as described in Hornstein (1990)
for English (Complex Tense Structures, CTS) and ex-
tended by Dorr and Gaasterland (1995) to cover in-
tervals, too. Since we envision independent modules
for the different sentence planning tasks that posit
their choices as constraints, the tense selection pro-
cess need not concern us.
Syntactic structure Possible values are pp
(prepositional phrase) and subord (subordinate
clause); both refer to the realization of the event that
acts as temporal reference point.
Quantification The lexicon contains the two val-
ues
quantifiable and not-quantifiable.
4.3 The shape of the lexicon
The possible values for the applicability and combin-
ability features can now be used in the lexicon to
describe individual temporal markers. Table 2 gives
the lexical representations for most of the German
anteriority markers and the posteriority marker be-
fore.
Similar representations have been developed
for the other marker classes given in table ]. si-
multaneity and posteriority. Feature values for in-
dividual markers have been identified by analysing
marker occurrences in the UMAS corpus (Glas 1975);
as such, they mainly reflect marker
usage.
We then
compared our marker descriptions to results from re-
search literature (see section 3). Note that combin-
ability conditions can apply to main and subordinate
clause/prepositional phrase separately, hence some
feature values are prefixed with me:, sc: and
pp:
to mark their scope. If a marker involves no con-
straint for a particular feature, the slot in the table
remains empty.
Table 2 contains an informal description of the lexi-
con entries; the formal representation depends on the
actual sentence planner used in text production, see
Grote and Stede (1998) for a preliminary proposal.
26
Feature
nachdem (after)
nach
(after)
sobald (as
soon as) kaum dab (no sooner bevor (before)
applicability
- denotation after(e,,, e~)A after(e,,, e,)A meets-i(e,~, e~) meets-i(e,~, e, ) before(e~, e~)A
meets-i(em, e,) meets-i(e~, e~) meets(e~, e,)
combinability
-
Aktionsazt
-
aspect
- tense
sc:resuitativeA
sc:iterativeA
sc:semelfactive
sc:perfective
mc:imperfective
E(e,)._E(e,n)_SA
E(e~)...E(e~), SA
E(e,)3_E(e,)
subord
pp:resultativeA
pp:iterativeA
pp:semelfactive
{E, S}(e~)
= {E, S}(e,)A
E(e~)_E(e.)_S
subord
E(e,)..E(e~)_SA
E(e,), E(e,,)_S
sc:non-durative
{E, S}(e~)
= {E, S}(e,)
- syntax pp subord subord
- quantifier quantifiable I quantifiable not-quantifiable not-quantifiable quantifiable
preferences
- style neutral brief neutral formal neutral
- intention evaluative
Table 2: Lexicon entries for some German temporal markers
4.4 Selecting temporal markers
This section briefly addresses the issue of selecting an
appropriate temporal marker during text production
using the discourse marker lexicon. We will focus on
the anteriority markers.
In our scenario, generation starts from a conceptual
representation which contains the facts that must be
reported in the text and their position in time. Let
us assume the following very simple input structure: 5
el:
arrive (he, home, 19 : 14)
e2:
watch(he ,TV, 19 : 15,22 : 30)
The first event precedes the second event, but they
'meet' at one point in time. Now, the first step is
to determine the applicable temporal relations. Two
interpretations are possible, depending on the dis-
course context and focus structure, which we have
not dealt with so far: Focussing on the earlier event
would yield the temporal relation meets(era, es), with
em = el, focussing on the later event the relation
meets-i(e,n, e,), with e,n = e2. Matching this against
the lexicon entries in table 2 would produce bevor in
the former case, and nachdem, nach, sobald, kaum
dat3 as candidate realizations for the latter interpre-
tation. Possible verbalizations are:
(8)
(a) Bevor
er
Fernsehen Euckte, ist
er navh Hause
gekommen.
(Before he watched TV, he has come home.)
(b) Sobald er nach Hause gekommen war, guckte er
Fernsehen.
(As soon as he had
come home,
he watched TV.)
(c) (Direkt) nachdem er nach Hause gekommen
war, gucckte er Fernsehen.
SThis
is an abridged representation.
We will eventually
represent the
facts as SitSpecs (Stede 1996), which will be an-
notated with temporal information. During lexicalization--as
one task in the sentence
planning phase---SitSpecs are mapped
onto semantic representations
(SernSpecs).
((Right) after he had come home, he watched TV)
(d) Nach dem Heimkommen guckte er Fernsehen.
(After coming home he watched TV.)
(e) Kaum dab er nach Hause gekommen war,
guckte er Fernsehen.
(As soon as he had come home, he watched TV.)
Assuming the anteriority interpretation (Sb-e),
how does a generation system choose among the four
remaining alternatives? We argued above that we
envision a modular architecture where independent
sentence planning modules posit their constraints re-
garding tense selection, lexicalization, syntactic real-
ization, etc. 6 In case no constraints are put forward
by the sentence planning modules, sobald (Sb) would
be selected, as it is the most specific and at the same
time neutral realization. If, however, a quantifier is
to be included, then nachdem would be chosen i~c).
If brevity is a stylistic concern, and the process in the
subordinate clause can be deverbalized, a phrasal re-
alization with the preposition nach is selected (Sd).
If, on the other hand, a more formal realization is
the overall goal given to the generator, kaum daft (Se)
would be chosen. In these cases, marker choice would
posit constraints (as given in the combinability slot
in table 2) on all other sentence planning decisions.
So far, we only considered a perfective aspect in
the subordinate clause. Once we change aspect to
imperfective, a realization including nachdem is no
longer an option, compare ,Nachdem er nach Hause
kam, hat er Fernsehen geguckt (After he came home,
he has watched TV). Sobald would be an adequate
realization. Likewise, changing the Aktionsart from
resultative to durative, as in Sobald er schlMt, guckt
6This approach differs from Dorr and Gaasterland (1995)
who impose a strict order on the selection of tense, aspect and
connecting word.
27
sie Fernsehen (As soon as he sleeps, she watches TV)
would rule out nachdem. With the resultative variant
einschlafen (fall asleep) both markers are possible.
Finally, if a constraint is posited that the tense has
to be 'present', kaum dab would not be available.
5 Conclusion and
Outlook
Temporal markers have neither received much atten-
tion in NLG, nor has a principled account of marker
selection as such been introduced. In this paper
we presented a general framework for representing
German temporal markers for generation purposes.
We identified some of the features required to de-
scribe applicability conditions, constraints and pref-
erences, and proposed a declarative lexical resource
that makes it possible to treat temporal markers and
other linguistic means as mutual constraints at the
sentence planning stage. Now, we need to examine
individual temporal markers more closely and incor-
porate the temporal marker lexicon into a text gen-
eration system.
For the purpose Of this paper, we have assumed
that temporal relations are always explicitly sig-
nalled, and thus limited our study to marker selec-
tion. Marker occurrence, however, is an important
issue. First, Hitzeman et al. (1995) argue that there
exist temporal defaults of the kind "An event will
occur just after a preceding event"; this renders the
introduction of explicit markers superfluous. Second,
we have only assumed pairs of time-stamped expres-
sions, but have ignored that they usually occur in
a larger discourse situation where other kinds of co-
herence relations might hold between events. For in-
stance, all causal coherence relations have some tem-
poral implicature; still, one does not want a temporal
marker to signal a VOLITIONAL-CAUSE, even though
cause and effect are temporally related.
Finally, future work needs to address the interac-
tion of marker choice and temporal adverbs, as these
are the means to realize the simple/progressive dis-
tinction in German.
Acknowledgement
Thanks to Manfred Stede and
two anonymous reviewers for helpful comments on
earlier versions of this paper.
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28
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