"My grandfather is my father's father."
Translation:Mijn grootvader is de vader van mijn vader.
I believe it's because they want the translation of 'father' specifically, instead of 'dad'. o uo
That's possible as well. The apostrophe is used in Dutch to prevent pronunciation issues:
- Piets boek = Piet's book
- Renés boek = René's book
- Theo's boek = Theo's book
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.