So far, our discussion of syntactic structure has tacitly assumed that all constituents in a given structure are overt. However, we now turn to argue that syntactic structures may also contain empty (= covert = null) categories – i.e. categories which have no overt phonetic form, and hence which are inaudible or silent. As we shall see, empty categories play a central role in the theory of grammar which we are outlining here.

We begin by looking at clauses which might be argued to contain an empty subject. In this connection, compare the structure of the bracketed infinitive clauses in the (a) and (b) examples below:

1. (1) (a) We would like [you to stay]

2. (b) We would like [to stay]

1. (2) (a) We don't want [anyone to upset them]

2. (b) We don't want [to upset them]

Each of the bracketed infinitive complement clauses in the (a) examples in (1–2) contains an overt (italicized) subject. By contrast, the bracketed complement clauses in the (b) examples appear to be subjectless. However, we shall argue that apparently subjectless infinitive clauses contain an understood null subject. (By saying that a constituent is null or empty or covert, we mean that it has no overt phonetic form and so is silent.) The kind of null subject found in the bracketed clauses in the (b) examples has much the same grammatical and referential properties as pronouns, and hence is conventionally designated as PRO.