"This is the house of the grandparents."
Translation:Das ist das Haus der Großeltern.
That works, too. People don't use it in spoken language, but it wouldn't be uncommon in formal writing or in a novel.
Why can't I say "von der Grosseltern" instead of just "der grosseltern"?
von requires the dative case, not the genitive -- so it would have to be von den Großeltern.
Because here it i-s a "sein sentence" where both sides r nouns, so both sides get nominative, unlike the sentence i asked about a few days ago (if somehow u remember it..), right?
For clarity: der Großeltern is genitive, but the entire noun phrase das Haus der Großeltern is nominative (hence the neuter nominative das).
I would disagree. It can depend on context or if you want to stress something. For example, "This is the house of the grandparents (whom) I was talking about".