Case refers to the form a noun or pronoun takes depending on its function in a sentence. English pronouns have three cases: subjective, objective, and possessive.
The subjective (or nominative) pronouns are I, you (singular), he/she/it, we, you (plural), they and who. A subjective pronoun acts as a subject in a sentence. See the sentences below for illustration:
The objective (or accusative) case pronouns are me, you (singular), him/her/it, us, you (plural), them and whom. (Notice that form of you and it does not change.) The objective case is used when something is being done to (or given to, etc.) someone. The sentences below show this use of the objective case:
There are two types of possessive pronouns. The first type is used with nouns my, your (singular), his, her, your (plural), its, their, our. The other type of pronouns are sometimes called independent possessive pronouns, because they can stand alone. They are mine, yours (singular), his, hers, ours, yours (plural) and theirs. The possessive pronouns show that something (or someone) belongs to someone (or something).