When looking at what the subject does in a sentence, you are looking at the action. A sentence consists of two basic parts: who/what is doing what. The cat (who/what) chased the mouse (did what). The subject is the who/what part of the sentence, and the verb is the action of the sentence. Consider the following sentence: He ran quickly. A pronoun is a word that renames the noun ("he" rather than "Jim" or some other proper name). And an adverb modifies a verb ("quickly" modifies "ran").