Why isn't the "pa" sentence accepted?
I believe it's because they want the translation of 'father' specifically, instead of 'dad'. o uo
What's the difference between "opa" and "grootvader"?
Opa is more commonly used and more informal. I don't know anyone who would still use 'grootvader', not even my father. It is more common though if you talk about your 'overgrootvader', your great-grandfather.
There is none. Grootvader is more polite/oldfashioned, might be more common in Flanders as well.
mijn opa is mijn vader's vader.. nothing wrong with that..
So, 'vaders vader' is a Dutch way of saying 'father's father'?
So Dutch uses the possessive 's' in a similar way to English, except that it doesn't use an apostrophe?
That's possible as well. The apostrophe is used in Dutch to prevent pronunciation issues:
Perhaps(?) because vader and grootvader are for speaking about relationship, and pa and opa are for referring directly to the persons.
Waarom niet: Mijn opa is de pa van mijn pa?