Constituent structure permits phrase structure rules to extend language by using recursion to embed one kind of constituent into another. By providing finite means for generating an unlimited number of sentences, recursive rules supply a crucial part of the answer to the fundamental question of the creative use of language. For example, proerties of particular words help determine syntactic well-formedness. For example, transitive verbs require direct objects, while such objects cannot be linked to intransitive verbs. The large lexical category V is thus divided into smaller lexical subcategories, each with its own special privileges of occurrence.
Surface structure is created by applying one or more transformation rules to the deep structure.
Transformation rules convert one phrase structure into another. But this fails., because the rule does not depend only on word order, but must be sensitive to more abstract categoires. This is fine, until you encounter a declarative sentence with more than one auxiliarry verb. But it additionally requires abstract structural analysis, of the sort discussed earlier in this chapter. A first glance might lead us to believe that quite a different rule is involved.