Ptát se na = Accusative?
Ptát se = Genitive?
You ask (of) someone (genitive) about something (accusative).
Why: "Ptaji si na tvoji babicku" is not accepted?
(Sorry for the accents!)
I just found out, that I should not put "si" but "se". Then, what is the difference between "si" and "se"?
Some verbs require si and some se and you have to remember which is which.
Technically, si is dative and se is accusative. It is not really helpful for most reflexive verbs.
why is wrong to use tvoji instead of vasi? sorry for no accents
It is not. You have to ask about a complete sentence. We need the exact copy to be able to say anything.
Sorry i'm talking about this sentence in particular. Oni se ptají na tvoji babičku? is this wrong?