A subject pronoun is a pronoun that is used as the subject of a sentence or clause. They include "I," "you," "he," "she," "it," "we," and "they." The role of a subject pronoun is to replace the noun that functions as the subject of the sentence. For example, instead of saying "Mary went to the store," you can say "She went to the store," with "she" replacing "Mary." To use subject pronouns correctly, make sure to match them with the correct verb form and subject agreement.