Picking Reference Events from Tense A Formal, Implement able Theory of English Tense-Aspect Semantics Trees:

Picking Reference Events from Tense A Formal, Implement able Theory of English Tense-Aspect Semantics Trees: Lenhart K. Schubert and Chung Hee Hwang Department of Computer Science, University of Rochester Rochester, New York 14627, U. S. A., and Department of Computing Science, University of Alberta Edmonton, Alberta, Canada T6G 2H1 Abstract. Despite numerous investigations into English tense-aspect semantics, the problem of formally inter- preting tense and aspect remains in large part unsolved. A formal solution requires (a) a well-defined mapping from a subset of English covering the most common tense-aspect constructions (including embedded tensed clauses) to a formal meaning representation, and (b) a well-defined denotational semantics for the meaning representation, which accords with speakers' intuitions about the original text. We propose a simple structure called a tense tree (or a set of connected tense trees) as a basis for interpreting tense-aspect constructions and some time adverbials in a context-dependent way. The de-indexicalization process simultaneously trans- forms tense trees and logical forms, the former in accord with simple recursion equations and the latter in accord with formal equivalences between context-indexed and context-independent logical forms. The rules have been implemented, and yield meaning representations in a for- mal episodic logic for narrative understanding. I. Introduction Narratives describe and relate episodes (events, situa- tions, eventualities). The episodes successively intro- duced appear to "line up" in systematic ways as a func- tion of the text structure, with tense and aspect playing a crucial role. (1-2) illustrate this familiar phenomenon: (1) John grinned at Mary. She frowned. (2) John knocked, but soon realized Mary had left. Each of the verbs in (1) appears to introduce an episode; the past inflection places them before the utterance event (or speech time), and the surface ordering suggests tem- poral sequencing, and probably a causal connection. In (2) we appear to have at least three episodes, a "knock- ing," a "realizing" and a "leaving." The past perfect aux- iliary relates the "leaving" to a past episode serving as reference point. (This reference point seems closely cor- related with the "realizing" episode, but on our theory is not identical with it.) As well, the adverb soon implic- itly references and relates the "knocking" and "realizing" episodes. The problem of automatically extracting these rela- tionships is complicated by a number of subtle issues: • How many episodes (events, situations, etc.) does a sentence implicitly introduce? We remarked that the introduction of episodes appears correlated with verbs; but in (3) John did not elude the tackle and score a touchdown, disappointing the fans. the causally relevant episode is characterized by the non-occurrence of successive "eluding" and "scor- ing" events; and in (4) When each guest congratulated Mary and gave her a present, she felt quite overwhelmed. the focus is on a "quantified" event consisting of any number of "congratulating" and "giving" events. What are the relative (semantic) scopes of tense and aspect, relative to NPs, VPs, adverbials, negation, etc.? Syntax suggests that tense scope is confined to the tensed verb or VP, yet …

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