12.3.6 Auxiliaries
The subclass of verbs called auxiliaries or helping verbs have particular syntactic constraints which can be viewed as a kind of subcategorization. Auxiliaries include the modal verbs can, could, may, might, must, will, would, shall, and should, the perfect auxiliary have, the progressive auxiliary be, and the passive auxiliary be. Each of these verbs places a constraint on the form of the following verb, and each of these must also combine in a particular order.
Modal verbs subcategorize for a VP whose head verb is a bare stem; for example, can go in the morning, will try to find a flight. The perfect verb have subcategorizes for
a VP whose head verb is the past participle form: have booked 3 flights. The progressive verb be subcategorizes for a VP whose head verb is the gerundive participle: am going from Atlanta. The passive verb be subcategorizes for a VP whose head verb is the past participle: was delayed by inclement weather.
A sentence can have multiple auxiliary verbs, but they must occur in a particular order: modal < perfect < progressive < passive. Here are some examples of multiple auxiliaries:
$$ \mathrm{m o d}a l\mathrm{p e r f e c t}\qquad\qquad\mathrm{c o u l d h a v e b e e n}a\mathrm{~c o n t e n d e r} $$
$$ \mathrm{m o d}a l\mathrm{p a s s i v e}\qquad\mathrm{w i l l}b e\mathrm{m a r r i e d} $$
$$ \mathrm{p e r f e c t~p r o g r o g s s i v e}\qquad\mathrm{h a v e~b e e n~f e a s t i n g} $$
modal perfect passive might have been prevented
Auxiliaries are often treated just like verbs such as want, seem, or intend, which subcategorize for particular kinds of VP complements. Thus can would be listed in the lexicon as a verb-with-bare-stem-VP-complement. One way of capturing the ordering constraints among auxiliaries, commonly used in the systemic grammar of Halliday (1985), is to introduce a special constituent called the verb group, whose subconstituents include all the auxiliaries as well as the main verb. Some of the ordering constraints can also be captured in a different way. Since modals, for example, do not have a progressive or participle form, they simply will never be allowed to follow progressive or passive be or perfect have. Exercise 12.8 asks the reader to write grammar rules for auxiliaries.
The passive construction has a number of properties that make it different than other auxiliaries. One important difference is a semantic one; while the subject of non-passive (active) sentence is often the semantic agent of the event described by the verb (I prevented a catastrophe) the subject of the passive is often the undergoer or patient of the event (a catastrophe was prevented). This will be discussed further in Ch. 18.