Pronoun case refers to the form a pronoun takes depending on its function in a sentence. Subjective case (I, he, she, we, they) is used when the pronoun is the subject; objective case (me, him, her, us, them) is used when the pronoun receives the action or follows a preposition; possessive case (my, his, her, our, their) shows ownership. English lost most of its noun case system centuries ago, but pronouns preserved it — making pronoun case one of the few places where grammatical function directly changes word form.
Simplify compound structures to test case: in "My brother and (I/me) went to the store," remove "My brother and" — "I went to the store" confirms the subjective case. Apply this reduction test consistently until correct case becomes intuitive.
You already know that pronouns substitute for nouns. What this topic adds is that English pronouns change their *form* based on what job they're doing — subject, object, or possessor. That's case. Most English nouns lost case endings centuries ago (we don't say "I gave the bookem to himster"), but pronouns held onto the system. This is why you can say "she hit him" but not "her hit he" — the form of the pronoun signals who is doing the action and who is receiving it.
The subjective case (I, you, he, she, it, we, they, who) is for pronouns that perform the action: they are the grammatical subject. "She laughed." "They arrived late." "Who is calling?" The objective case (me, you, him, her, it, us, them, whom) is for pronouns that receive the action, or that follow prepositions. "He called me." "The award went to her." "Whom did you call?" The possessive case (my/mine, your/yours, his, her/hers, its, our/ours, their/theirs, whose) shows ownership: "That is my book." "The decision is hers." Note that possessive pronouns never use an apostrophe — "its" is the possessive, "it's" means "it is."
The hardest errors arise in compound constructions: "My sister and (I/me) went to the store." When you're uncertain, mentally strip the compound down to just the pronoun. "I went to the store" vs. "Me went to the store" — only *I* sounds right, confirming subjective case. The reverse works too: "He gave the tickets to my sister and (I/me)." Strip to: "He gave the tickets to me" vs. "He gave the tickets to I." The objective *me* wins. This reduction test solves almost every compound case problem automatically.
Prepositions always take the objective case — every preposition in English: to, from, for, with, between, among, at, about. This is why "between you and me" is correct and "between you and I" is hypercorrection. People say "between you and I" trying to sound formal, but they're overcorrecting based on the (correct) observation that subjects use *I*. The mistake is not remembering that prepositions are not verbs: they govern objects, not subjects. "For her and me," "with us," "from him and them" — the object form follows every preposition, no exceptions.