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.

(i) [+Tense] [+Agr] INFL may dominate modal auxiliaries, periphrastic do, or auxiliary have and be
(ii) [-Tense] [-Agr] INFL may dominate to.
(Other non-finite constructions are: infinitives without to, participial constructions and gerunds.)

(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:

Subjects of non-finite clauses have accusative case:
John believed [her to be in love with him]

Subjects of finite clauses have nominative case:
John believed [that she was in love with him]

(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.

Morphological properties
Thematic properties
Syntactic properties

Exercises on properties of INFL