INFL as a functional head
So far, we have seen that the verb plays a major part in the organisation of the sentence, e.g. by determining the argument structure (and thus, the type of subject and the number and type of complements). However, in this section we will see that the VP is not the top-most node that represents the sentence. This becomes evident by looking at examples that contain more than one verbal element, e.g. a combination of an auxiliary and a lexical verb (cf. 1 below) or a combination of a modal and a lexical verb (cf. 2 below).
(1) Anne has bought a new car.
(2) Anne will/might buy a new car.
The examples illustrate that there must be a position for auxiliaries and modals outside the VP. We can further see that the subject occurs in a position to the left of modals and auxiliaries. This suggests that the subject, though base-generated in the specifier, moves to another position.
In this section we suggest that there is a functional category INFL (projecting to a phrase called IP) which takes the VP as its complement. Modals and auxiliaries may be residents of INFL (i.e. the head position) and the subject of the sentence moves to the specifier position of the IP.
Motivation for assuming an INFL node:
I. Finiteness is a feature of the clause that is often marked on a main verb or auxiliary by inflectional morphology.
(a) Clauses can be finite or non-finite.
(b) Verbs may take clausal complements, but they differ in whether they take complements in the form of:
II. In English, finiteness is associated with tense, which in turn seems to be associated with the verb.
(a) Past tense verb forms can appear only in finite clauses:
(b) Inflectional marking that normally appears on the main verb may appear outside the VP:
The fact that the tense features can be separated from the VP shows that finiteness / tense is not a property of V° but of some other category.
(c) If we assume that the syntactic features of the whole phrase originate on the head, then we need to find a head to bear the feature responsible for the distinction between finite and non-finite (or tensed and tenseless) clauses.
III. We thus hypothesize INFL, which bears the functional features tense ([Tense]) and agreement ([Agr]) or closed class auxiliaries that realize these features (cf. IV. below).
Because it is associated with these elements rather than open class lexical heads, which bear the lexical features N and V, INFL is a non-lexical or functional head.
IV. Certain closed-class items may be "residents" of INFL:
(a) in finite clauses: modal auxiliaries, auxiliary have and be, periphrastic do (as well as inflectional morphemes (e.g. past tense -ed) although they are on the verb)
(b) in non-finite clauses: the infinitive marker to