A sentence is a grammatically complete group of words that expresses a thought. In standard English, a grammatically complete sentence must contain a subject, which identifies what the sentence is about, and a predicate, which says or asks something about the subject or tells the subject to do something.

Some sentences have only a one-word predicate with an implied, or understood, subject (for example, Stop!). Most sentences, however, contain additional words that expand the basic subject and predicate. In the preceding example, for instance, the subject might have been simply Stephen Colbert; the words who hosts a cable TV show tell us more about the subject. Similarly, the predicate of that sentence could grammatically be pretends; the words to be a conservative expand the predicate by telling us what Colbert pretends.