I know you asked this a year ago, but in case anyone else reading can't follow the other comments...
"l'uva" is closer in meaning to "the bunch of grapes" in English. Grammatically the Italians treat an amount grapes like a single compound fruit, una uva. A single grape in English would be un chicco d'uva (litterally something like "a grape grain/berry"). There is also the word "acino" which is used instead of "chicco" with a similar meaning.
So, "È l'uva" would translate to "It's the bunch of grapes" or just "it's the grapes" It's still a weird sentence, but yes, the singular "l'uva" in Italian becomes the plural "the grapes" in English.
eh, not really. Deer is just an irregular plural; like many animal names in English (especially game animals), it's the same whether you're referring to one or several of them.
Italian uva is maybe more like grass. If you want to refer to a single unit of grass, you say a blade of grass. An entire yard is still just grass -- grasses is usually something different, like the entire collection of related plants. See, for example, the caption for the photo on the right at https://en.wikipedia.org/wiki/Poaceae#Ecology -- "A kangaroo eating grass".
L' is the article before nouns that start with a vowel. L'elefante (masculine) l'acqua (feminine). For nouns that start with a consonants, La is feminine and il and lo are the masculine ones. You use lo is the noun starts with s + another consonant (lo studente), the noun starts with a z (lo zaino) or y (lo yogurt).
I had "mangio l'uva" and was marked as incorrect. I thought the pronoun was optional (I've been doing this already on Duolingo and it just shows me an "alternative solution" that includes the pronoun).
My Italian teacher in college told us that the pronoun was often left off, since the verb indicates it already. You really only include the pronoun for emphasis.
I've reported my answer as "Correct." But, am I wrong?
I am not sure here, but my impression is that Italian essentially treats l'uva as an uncountable noun. If that is correct, than le uve would be only used to talk about different varieties. Sort of like saying exotic teas or California wines. It would essentially change the meaning.