Syntax
The structure of IP
Example 1: IP as the head of sentences
Sally saw a black cat.
In this example INFL is specified as [+tense] and [+agr], i.e. we are dealing with a finite clause. The subject DP Sally has moved from the specifier-position of VP (cf. ti) to the specifier-position of IP.
Example 2: Embedded non-finite IP with overt subject
Peter wanted [IP Mary to drive the car].
The second example is a complex sentence, consisting of a finite main clause (the IP Peter wanted + clausal complement) and a non-finite embedded clause (the IP Mary to drive a car). In the main clause INFL is specified as [+tense] and [+agr], and the subject DP Peter has moved from the specifier-position of VP (cf. ti) to the specifier-position of IP. In the embedded clause INFL is specified as [-tense] and [-agr], and it is lexically filled with the infinitive marker to. The subject DP Mary has moved from the specifier-position of VP (cf. tk) to the specifier-position of IP.
Example 3: Embedded non-finite IP with PRO-subject
The student planned [IP PRO to write a term paper].
The third example is again a complex sentence, where a non-finite IP is the complement of the main clause verb planned. In the embedded clause INFL is specified as [-tense] and [-agr], and it is lexically filled with the infinitive marker to. The subject is not overtly realised, but interpreted as co-referential with the main clause subject the student (this is why both subjects bear the index i). It is represented as PRO, which - like all other subjects - has moved from the specifier-position of VP (cf tj) to the specifier-position of IP illustrated by tk).