- A nominative generally is the subject, or actor, of the verb. Nominative pronouns are I, he, she, we, they, and who.
- An accusative generally is the object, or receiver, of the verb. Accusative pronouns are me, him, her, us, them, and whom.
- Genitive is primarily a way to indicate possession; it's a noun that modifies another noun. Genitive case is marked on pronouns: my, your, his, her, our, their, whose, its and on noun phrases with 's.
Another tricky one for me is who versus whom. But the nominative/accusative difference should pin it down. Who kissed the bride? Whom did Henry kiss?
What grammar issues do you find tricky?