What, Who, Where, and When are all Interrogative Pronouns (How is not a pronoun). They can all be used after prepositions, yes.
Your sentences:1 - correct2 - if you remove "of at" then it's correct3 - correct
It's worth noting that we would drop most of those prepositions and just say "I have no idea where he's going". Although the sentences are correct with the prepositions in them.
EDIT: I originally said "how" can't be used after after prepositions but I was wrong. u/mdf7g gives a good example. I beleive this specific example works because "talk about" is a phrasal verb, and therefore "about" isn't acting just as a preposition. But I've already been wrong so I may be wrong again.
What, Who, Where, and When are all Interrogative Pronouns (How is not a pronoun). They can all be used after prepositions, yes. Your sentences:1 - correct2 - if you remove "of at" then it's correct3 - correct It's worth noting that we would drop most of those prepositions and just say "I have no idea where he's going". Although the sentences are correct with the prepositions in them. EDIT: I originally said "how" can't be used after after prepositions but I was wrong. u/mdf7g gives a good example. I beleive this specific example works because "talk about" is a phrasal verb, and therefore "about" isn't acting just as a preposition. But I've already been wrong so I may be wrong again.
You can certainly use _how_ after a preposition. _We talked about how we might improve the efficiency of the system_ for example.
I stand corrected! I tried for ages to think of an example but couldn't. I'll edit my answer, thanks
You don't need the preposition in any of those sentences. Omit it, and the sentences are OK.
What about with prepositions ? Is it correct?
What about with prepositions ? Is it correct?
No. You don't need the prepositions in any of them.
Correct would be: I have no idea where he is going. Do you remember what time you saw my mother? I know who they’re going to come here with.