Pronouns have three cases - subjective, objective, and possessive. The subjective case is used when the pronoun is the subject or subject complement. The objective case is used when the pronoun is the direct or indirect object. The possessive case indicates ownership and is formed using an apostrophe. Who and whoever are used as subjects, while whom and whomever are used as objects. Pronoun case must agree with its use in compounds, elliptical constructions, and with the pronoun's antecedent for clear understanding.