Representing Temporal Discourse Markers For Generation Purposes

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 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 planning stage. We illustrate how this resource can be used to produce alternative verbalizations of the temporal relationship holding between two events. 1 M o t i v a t i o n In text, discourse markers signal the kind of coherence 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 correct 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 i n the text does not correspond to their order of occurence: In example (1), nachdem (after) marks the event denoted in the second clause as temporally anterior to the one denoted in the first clause. In example (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 straightforward matter, though. First, one and the same temporal 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 syntactic 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 prepositions. Second, temporal discourse markers can even overwrite the temporal relations indicated by other linguistic means like tense and aspect, as noted by researchers who work in the analysis of temporal markers (e.g. Hitzeman et al. 1995). In a nutshell, when aiming at selecting an appropriate temporal marker in text generation one needs a representation of temporal markers that enables marker choice and other sentence level decisions (such as tense and aspect selection) to mutually constrain 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 sentence 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 reviews related work on generating temporal markers. Section 3 describes the major semantic, pragmatic and syntactic properties of German temporal prepositions and subordinating conjunctions. Section 4 presents the generation perspective: It briefly discusses the shape of the discourse marker lexicon, introduces the features used in the lexicon, and presents sample lexicon entries and their application in the generation process.