Can someone explain, why I have to use an article before nostre here, but some questions before in the sentence "il gatto è nostro" there was no article necessary? i can't figure out what the difference is, because i don't think it has something to do with the plurars. I unterstand that there is no article needed in case of close family members. so is a cat a family member?
It's "nostre" that suggests that the apples are ours. You state that the sentence can be written with or without the article but when I wrote "Il coltello è il nostro" on a previous question it was marked wrong and the comments explained that one is to exclude the article when the possessive is written without a noun (not a combination). "Le nostre mele" but "Le mele sono nostre." ????? Because it's plural?
As I see it, and I could be wrong, the absence or presence of the article with the possessive pronoun changes the connotation.
Without the article, it addresses who the owner is:
"Le mele... sono nostre" might be analysed as "There are some apples and they belong to us (not to someone else)".
The sentence with the article addresses which ones belong to someone.
"Le mele nel piatto sono
lenostre" might be analysed as "our apples are the ones on the plate (not those other ones)"
Comments and corrections welcome.