Every complete sentence contains two parts: a subject and a predicate. The subject is what (or whom) the sentence is about, while the predicate tells something about the subject. In the following sentences, the predicate is enclosed in braces ({}), while the subject is highlighted.

Judy {runs}.

Judy and her dog {run on the beach every morning}.

To determine the subject of a sentence, first isolate the verb and then make a question by placing "who?" or "what?" before it -- the answer is the subject.

The audience littered the theatre floor with torn wrappings and spilled popcorn.

The verb in the above sentence is "littered." Who or what littered? The audience did. "The audience" is the subject of the sentence. The predicate (which always includes the verb) goes on to relate something about the subject: what about the audience? It "littered the theatre floor with torn wrappings and spilled popcorn."