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.
You can but there is no option for that
Why not "des" Großeltern???
Because Großeltern is plural, not masculine or neuter.
So you need plural genitive der, not masculine/neuter genitive des.
But not "Dies ist das Haus"?
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 do people not use "dies" in spoken language?
because they don't
I would guess because it seems formal and people prefer to use an informal equivalent
The English is really stilted (reported). It is much more common to say "This is the grandparents' house."
Agree. This is wonky English
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".