Adjectives

In addition to being modifiers, adjectives in the XTAG English grammar can be also anchor clauses (see Adjective Small Clauses in Chapter 9). There is also one tree family, Intransitive with Adjective (Tnx0Vax1), that has an adjective as an argument and is used for sentences such as Seth felt happy. In that tree family the adjective substitutes into the tree rather than adjoining as is the case for modifiers.

  

Figure: Standard Tree for Adjective modifying a Noun: $\beta$An

As modifiers, adjectives anchor the tree shown in Figure 19.1. The features of the N node onto which the $\beta$An tree adjoins are passed through to the top node of the resulting N. The null adjunction marker (NA) on the N foot node imposes right binary branching such that each subsequent adjective must adjoin on top of the leftmost adjective that has already adjoined. Due to the NA constraint, a sequence of adjectives will have only one derivation in the XTAG grammar. The adjective's morphological features such as superlative or comparative are instantiated by the morphological analyzer. See Chapter 22 for a description of how we handle comparatives. At this point, the treatment of adjectives in the XTAG English grammar does not include selectional or ordering restrictions. Consequently, any adjective can adjoin onto any noun and on top of any other adjective already modifying a noun. All of the modified noun phrases shown in ((287))-((290)) currently parse with the same structure shown for colorless green ideas in Figure 19.2.

(286)0(286

(287)

big green bugs  (287)0(287

(288)

big green ideas  (288)0(288

(289)

colorless green ideas  (289)0(289

(290)

?green big ideas 

  

Figure 19.2: Multiple adjectives modifying a noun

While ((288))-((290)) are all semantically anomalous, ((290)) also suffers from an ordering problem that makes it seem ungrammatical in the absence of any licensing context. One could argue that the grammar should accept ((287))-((289)) but not ((290)). One of the future goals for the grammar is to develop a treatment of adjective ordering similar to that developed by [#!ircs:det98!#] for determiners^19.2. An adequate implementation of ordering restrictions for adjectives would rule out ((290)).