"Are the grapes his or ours?"
Translation:האם הענבים שלו או שלנו?
The meanings of both sentences are very similar in both Hebrew and English, but I would translate "his grapes or our grapes" as הענבים שלו או (הענבים) שלנו? It's hard to explain the difference, and it's not something I learned, it's a speaker's intuition, but I'll try.
(האם) הענבים (הם) שלו או שלנו?
Derived from "the grapes are his". Turned into a question: are the grapes his? or ours? הענבים הם שלו. הענבים הם שלו? או שלנו? The הם is a copula and optional, and the האם is the question word and is also kind of optional, formally it's preferred. In daily use I would usually omit it.
הענבים שלו או (הענבים) שלנו?
Derived from "his grapes". Not a sentence, just a noun phrase. But sometimes we use fragments instead of full sentence when the context is clear. - The grapes are good, aren't they? - Which grapes? His grapes? הענבים שלו. הענבים שלו? או שלנו? And you could say הענבים שלו או הענבים שלנו? but the second הענבים is optional. האם doesn't come into it at all, I guess because it's a fragment and not a full sentence.
Annoyingly, when you form the two sentences, and go for the leanest option on both, they end up looking the same. Bother.