So far I have suggested in Chapter 1 that auxiliaries show the prototypicality structuring typical of word classes, and in Chapter 2 I have argued that their grammatical behaviour reflects their lack of verbal morphosyntax. The argument was informal, and the claim is presumably compatible with a range of linguistic theories, at least in the sense that they do not preclude such an analysis. In this chapter I will underpin my argument by showing that there is a simple and coherent account of the type of lexical structuring of auxiliaries which I have posited, in which relevant generalizations are captured, within the formal framework of Head-driven Phrase Structure Grammar (HPSG). This includes an account of the distribution of periphrastic do, which I shall suggest is found when tense cannot be realized as an affix because it has some characteristic which is properly that of a word, thus providing an answer to question 7 of the list at the beginning of my §1.5. The discussion will necessarily be somewhat technical, and some readers may prefer to take the demonstration of this chapter for granted, and proceed directly to Chapter 4; I have recapped the small amount of discussion that is relevant to succeeding chapters so that the book coheres if it is read in this way.

In adopting HPSG I do not, of course, wish to suggest that it provides the only possible formalization of the account of Chapter 2.