who, which, what, etc after prepositions

Interrogative conjunctions like who, which, what, where, whether can follow prepositions.

• We discussed the question of who to appoint as manager.
• I may be able to come: it depends on which day you are holding the meeting.
• They were not at all interested in what I thought.

After common words like tell, ask, idea, prepositions are often dropped before these conjunctions.

• Tell me about your trip. but Tell me where you went.
• Do you have any idea of the cost? but Do you have any idea what it costs?

Prepositions are not normally used before other conjunctions such as that. Either they are dropped:

• I knew about his problems. but I knew that he had problems. (not I knew about that he had problems.)

or the expression the fact is put between the preposition and that.

• I was not responsible for the fact that the documents were stolen. (not I was not responsible for that the documents were stolen.)

note

For more details, (see here).