The document discusses the subject and predicate in sentences. The subject is the doer of the action in a sentence, typically a noun or pronoun, that comes before the predicate. The predicate contains the verb and any other elements and describes the action performed by the subject. Some examples are provided to illustrate identifying the subject and predicate in different types of sentences. The key difference between a subject and predicate is that the subject is who or what the sentence is about, while the predicate describes the action done by the subject.