A subject pronoun is a word that replaces a noun in a sentence and acts as the subject. Common subject pronouns include I, you, he, she, it, we, and they. These pronouns help avoid repetition and make sentences clearer. For example, instead of saying "John is going to the store," you can say "He is going to the store." Subject pronouns are essential in English grammar because they indicate who is performing the action of the verb. They are used in various contexts, such as in questions and statements. Understanding subject pronouns is crucial for constructing grammatically correct sentences.