It depends on whether the pronoun is the subject (doing the verb) or the object (receiving the verb):
- Subject → "you and I": You and I should grab lunch. (You and I = subject of "should grab")
- Object → "you and me": Send the file to you and me. (you and me = object of "to")
The quick test: drop "you" and see which sounds right alone.
- ...and I should grab lunch → "I should grab lunch" ✅
- ...and me should grab lunch → "Me should grab lunch" ❌
The mistake that gave "you and I" a bad reputation is overcorrection — people say "between you and I" because they think "me" sounds informal, but "between you and me" is actually correct (object of the preposition "between").