Having said that, I can't believe the number of times in a single day that I hear knowledgeable people, especially television newscasters and commentators who should've mastered basic grammar in college or elementary school, screw up their pronouns.
*Simply put, a nominative pronoun is the subject of the sentence: I, you, he, she, it, we, they, who, and whoever.
Ex: She and Mark like ice cream. Not - Her and Mark like ice cream.
Ex: He and I went to the party. Not - Him and me went to the party.
Ex: Who is going? Not - Whom is going?
Two subjects can make it a little tricky so take out one of the subjects and listen to the sentence. If you use the wrong pronoun, it sounds weird and you'll know it.
Not - Her like ice cream. Not - Me went to the party.
*Objective pronouns are used with direct objects, indirect objects, and as objects of prepositions: me, you, him, her, it, us, them, whom, and whomever.