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<Paper uid="W98-0304">
  <Title>Representing temporal discourse markers for generation purposes</Title>
  <Section position="5" start_page="22" end_page="24" type="metho">
    <SectionTitle>
3 Linguistic perspective:
</SectionTitle>
    <Paragraph position="0"> 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 markers. This section introduces the major properties and explores the correlations between temporal markers and other linguistic means that indicate temporal organization. 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).</Paragraph>
    <Section position="1" start_page="22" end_page="23" type="sub_section">
      <SectionTitle>
3.1 The 'meaning' of German temporal
markers
</SectionTitle>
      <Paragraph position="0"> Temporal subordinating conjunctions and temporal prepositions conjoin two events where the event in the subordinate clause (or the PP) provides the temporal framework for interpreting the event in the main clause: Bevor Sie den Toaster reinigen, den Netzstecker ziehen. (Before you clean the toaster, unplug the device.) and the corresponding 'shorthand' form l br dem Reinigen des Toasters den Netzstecker ziehen (Unplug before cleaning the toaster).</Paragraph>
      <Paragraph position="1"> Semantic properties German grammars such as Helbig and Buscha (1991) list about 20 temporal subordinating conjunctions and 20 temporal prepositions. 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 either overlap with (simultaneity), succeed (anteriority), or precede (posteriority) the event depicted in the subordinate clause or the prepositional phrase.</Paragraph>
      <Paragraph position="2"> In table 1 we provide a synthesis of the classifications of the most frequent German temporal markers by Helbig and Buscha (1991), Buscha (1989) and B/iuerle (1995). The markers listed in the table reflect the scope of the marker study in this paper.</Paragraph>
      <Paragraph position="3"> 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 meaning aspect. Consider the markers of simultaneity: Solange, for instance, conveys the idea of a strict simultaneity 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.</Paragraph>
      <Paragraph position="4"> Second, table 1 shows that some markers are ambiguous: Als and wenn occur in all three classes, seitdem, 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.</Paragraph>
      <Paragraph position="5"> 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 'leaving the house'. They differ in that they focus on  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)  relations. Note that the corresponding English markers are only approximate translations. different aspects of the situation: In (3a) the earlier 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 accommodation (Lascarides and Oberlander 1993). However, the treatment of the discourse behaviour of temporal markers is beyond the scope of this paper.</Paragraph>
      <Paragraph position="6"> Pragmatic issues further concern style. Regarding temporal markers, stylistic features are of minor importance: We only observe variation between archaic and neutral (da vs. als), and formal and neutral (kaum dab vs. sobald) markers.</Paragraph>
    </Section>
    <Section position="2" start_page="23" end_page="24" type="sub_section">
      <SectionTitle>
3.2 Syntactic and lexical constraints
</SectionTitle>
      <Paragraph position="0"> 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.</Paragraph>
      <Paragraph position="1"> Conversely, these contexts can influence the meaning of markers. 1 Markers and Aktionsart/aspect Aspect is traditionally taken to have two components, the non-inherent grammatical features, and the inherent lexical 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 features 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), semelfactive (klopfen/to knock), resultative (verbrennen/to burn up) and causative verbs (trii.aken/to water).</Paragraph>
      <Paragraph position="2"> 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.</Paragraph>
      <Paragraph position="3"> Two kinds of interdependencies are generally acknowledged, see Ehrich (1987), Buscha (1989) and B~iuerle (1995). First, temporal markers are sensitive 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 subordinate 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.</Paragraph>
      <Paragraph position="4"> (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 perspective a speaker adopts with respect to an event, such as perfective (temporally closed) or imperfectire. In German, this distinction is grammatically realized by choosing a perfective or simple tense, e Aktionsart and aspect closely interact, consider example (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.</Paragraph>
      <Paragraph position="5"> (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 progressive and simple cannot be signalled by morphological features of the verb, but require a separate temporal adverb: He is reading vs. Sie 1lest gerade (She reads right now).  Here, verb properties determine the reading of the temporal marker. Our study of temporal marker occurrences in the LIMAS corpus suggests that markers belonging to the simultaneity class typically realize imperfective aspect, whereas temporal connecting words that signal anteriority correlate with a perfective aspect in the subordinate clause.</Paragraph>
      <Paragraph position="6"> 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 correlates with present and past tense:  (7) (a) Als er in Dresden war (*ist), suchte (,sucht) er seine 1~reundin auPS (b) Wenn er in Dresden ist / war, sucht / suchte er seine Freundin auPS  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. Assuming the Reichenbachian threefold distinction between Event Time (E), Reference Time (R), and Speaking Time (S) (the Basic Tense Structure, BTS, (Reichenbach 1947)), we observe that the constraints imposed by a marker on verb tense concern the underlying relation 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.</Paragraph>
      <Paragraph position="7"> Markers and syntactic structure The most straightforward correlation is that between syntactic 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 (temporal) prepositional phrase is realized.</Paragraph>
      <Paragraph position="8"> 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).</Paragraph>
    </Section>
  </Section>
  <Section position="6" start_page="24" end_page="27" type="metho">
    <SectionTitle>
4 Generation perspective:
</SectionTitle>
    <Paragraph position="0"> Representing temporal markers A representation of temporal markers suitable for generation purposes has to accommodate the following demands: First, it has to describe the semantic SThe comma stands for 'is cotemporal', the underscore for 'precedes'.</Paragraph>
    <Paragraph position="1"> 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 markers 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 assembles specifically the information associated with discourse markers.</Paragraph>
    <Paragraph position="2"> Traditional lexicology and grammars describe lexical entries along three features: semantic, pragmatic and syntactic dimensions (see section 3). From the production perspective, these features are to be classifted with respect to when and where they come into play in the generation process; this amounts to a procedural 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 conditions that need to be present in the input representation for the marker to be a candidate.</Paragraph>
    <Paragraph position="3"> Chiefly, this is the semantic/discourse relation to be expressed, and also (if applicable) features pertaining to presuppositions and intentions.</Paragraph>
    <Paragraph position="4"> * Combinability conditions: The constraints that the marker imposes on its neighbouring linguistic constituents (the 'syntagmatic' dimension). These are syntactic constraints on subcategorization and semantic type constraints, which interact with other realization decisions in sentence planning.</Paragraph>
    <Paragraph position="5"> * Distinguishing features: If preferential choice dimensions, such as style, brevity, etc., are attended to in the system, then these features serve to distinguish markers that are otherwise equivalent (the 'paradigmatic' dimension).</Paragraph>
    <Paragraph position="6"> In the remainder of this section we describe lexicon entries for temporal markers along these lines.</Paragraph>
    <Section position="1" start_page="24" end_page="25" type="sub_section">
      <SectionTitle>
4.1 Applicability conditions
</SectionTitle>
      <Paragraph position="0"> Semantic conditions The semantic classes introduced in section 3.1 (simultaneity, anteriority and posteriority) turned out to be too coarse for generation 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.</Paragraph>
      <Paragraph position="1"> Allen's temporal interval relationships provide an adequate framework (Allen 1984), as already sug- null gested by Dorr and Gaasterland (1995). Allen introduces seven basic temporal interval relationships, namely equals(=), after(&gt;), during(d), overlaps(o), meets(m), starts(s), finishes(f)--and their inverses &lt;,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.</Paragraph>
      <Paragraph position="2"> Each temporal relation corresponds to one or several German temporal markers, for instance, overlaps may be expressed by the entire range of simultaneity markers given in table 1, except for solange and kaum daB. Conversely, the majority of the temporal markers can realize several temporal interval relations. 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 ).</Paragraph>
      <Paragraph position="3"> This adequately captures the semantic difference between w&amp;hrend and solange. In the lexicon, the applicability conditions of a particular temporal marker are now described by listing the temporal interval relations it can realize.</Paragraph>
      <Paragraph position="4"> Pragmatic conditions In section 3.1 we briefly discussed pragmatic features of temporal markers.</Paragraph>
      <Paragraph position="5"> For the time being, the lexicon supports the features style, with the values neutral, brief, formal, archaic, and intention. Its value evaluative indicates the speaker's (negative) attitude towards the kind of temporal relation holding between two events (Steube 1980; Buscha 1989).</Paragraph>
    </Section>
    <Section position="2" start_page="25" end_page="25" type="sub_section">
      <SectionTitle>
4.2 Combinability conditions
</SectionTitle>
      <Paragraph position="0"> 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 Aktionsart. 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 Aktionsarten in German (see also section 3.2). 4 At present, the lexicon supports a subset of Bussmann's Aktionsarten, namely stative, durative, iterative, semelfactive, causative and resultative.</Paragraph>
      <Paragraph position="1"> 4There is no generally accepted and well-defined set of Aktionsart features; we opted for Bussmann (1990) because these features are supported by the lexicalization component we intend to use (Stede 1996).</Paragraph>
      <Paragraph position="2"> Aspect Grammatical aspect is encoded using the feature values perfective and imperfective.</Paragraph>
      <Paragraph position="3"> 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 grammatical tenses that meet this constraint.</Paragraph>
      <Paragraph position="4"> Mapping this representation into grammatical tense requires knowledge on how to map pairs of Basic Tense Structures to the tense structure of complex German sentences, as described in Hornstein (1990) for English (Complex Tense Structures, CTS) and extended by Dorr and Gaasterland (1995) to cover intervals, too. Since we envision independent modules for the different sentence planning tasks that posit their choices as constraints, the tense selection process need not concern us.</Paragraph>
      <Paragraph position="5"> 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.</Paragraph>
      <Paragraph position="6"> Quantification The lexicon contains the two values quantifiable and not-quantifiable.</Paragraph>
    </Section>
    <Section position="3" start_page="25" end_page="26" type="sub_section">
      <SectionTitle>
4.3 The shape of the lexicon
</SectionTitle>
      <Paragraph position="0"> The possible values for the applicability and combinability 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 before. Similar representations have been developed for the other marker classes given in table \]. simultaneity and posteriority. Feature values for individual 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 research literature (see section 3). Note that combinability 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 constraint for a particular feature, the slot in the table remains empty.</Paragraph>
      <Paragraph position="1"> Table 2 contains an informal description of the lexicon entries; the formal representation depends on the actual sentence planner used in text production, see Grote and Stede (1998) for a preliminary proposal.</Paragraph>
      <Paragraph position="2">  - syntax pp subord subord - quantifier quantifiable I quantifiable not-quantifiable not-quantifiable quantifiable preferences - style neutral brief neutral formal neutral - intention evaluative</Paragraph>
    </Section>
    <Section position="4" start_page="26" end_page="27" type="sub_section">
      <SectionTitle>
4.4 Selecting temporal markers
</SectionTitle>
      <Paragraph position="0"> 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.</Paragraph>
      <Paragraph position="1"> 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 discourse 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 interpretation. 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.</Paragraph>
      <Paragraph position="2"> (As soon as he had come home, he watched TV.) (c) (Direkt) nachdem er nach Hause gekommen  war, gucckte er Fernsehen.</Paragraph>
      <Paragraph position="3"> SThis is an abridged representation. We will eventually represent the facts as SitSpecs (Stede 1996), which will be annotated with temporal information. During lexicalization--as one task in the sentence planning phase---SitSpecs are mapped onto semantic representations (SernSpecs).</Paragraph>
      <Paragraph position="4"> ((Right) after he had come home, he watched TV) (d) Nach dem Heimkommen guckte er Fernsehen.</Paragraph>
      <Paragraph position="5"> (After coming home he watched TV.) (e) Kaum dab er nach Hause gekommen war, guckte er Fernsehen.</Paragraph>
      <Paragraph position="6"> (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 regarding tense selection, lexicalization, syntactic realization, 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 realization 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.</Paragraph>
      <Paragraph position="7">  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.</Paragraph>
    </Section>
  </Section>
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