Some properties of INFL
By X-Bar theory, INFL is the head of IP, which is the sentence.
Example 1: Finite IP
Anne will buy a new car.
(a) INFL is always specified for [Tense] (and [Agr]), and may (but need not) be filled by a word.
(b) The specification of INFL as [+Agr] does not mean that agreement marking will be realized, and in fact in English it is only in present 3rd person singular verb forms:
she smiles/* she smile
(c) The specification of INFL as [+Tense] or [-Tense] (or perhaps [+Agr] or [-Agr]) determines whether a subject will have nominative or accusative case:
(d) Under certain conditions, subjects in non-finite clauses can also have no overt linguistic realisation. Such subjects are represented by the placeholder PRO. They are interpreted implicitly as co-referential with a DP of the main clause (i.e. the interpretation of PRO is "controlled" by a DP in the main clause, cf. Control Theory in the framework of Government & Binding Theory):
Anne wanted [PRO to buy a car] (PRO=interpreted as Anne)
Example 2: Non-finite IP
Anne wanted to buy a car.
Exercises on properties of INFL