A pronoun is a part of speech that can be used to replace a noun. There are many different kinds of pronouns: personal, reflexive, demonstrative, relative, interrogative, and indefinite.
Case refers to the way pronouns are used in a sentence, and this can be subjective, objective, or possessive. Choosing between the subjective and objective case can be confusing; for example, the choice between who or whom. Pronouns must clearly refer to their antecedents and the nouns they represent; they must also agree with their antecedents in number and gender. If you can use pronouns skillfully, your writing will be clearer.