Prepositions are not usually used directly before conjunctions, although it is possible in some cases.

Before that

Prepositions are not usually used directly before the conjunction that. In indirect speech, prepositions are usually dropped before that-clauses.

Compare:

I knew about his financial problems.
I knew that he had financial problems. (NOT I knew about that he had financial problems.)
I had no idea of his condition.
I had no idea that he was in trouble. (NOT I had no idea of that he was in trouble.)
I was surprised at her intelligence.
I was surprised that she was so intelligent. (NOT I was surprised at that she was so intelligent.)

In other cases the expression the fact is put between the preposition and that.

The judge paid no attention to the fact that the accused was in love with the woman he allegedly killed.

(NOT The judge paid no attention to that the accused …) (NOT The judge paid no attention that the accused was in love…)

Question words

Prepositions are usually dropped before the question words who, which, what etc. This usually happens after the words tell, ask, depend, sure, idea, look etc.

Tell me about your vacation.
Tell me where you went. OR Tell me about where you went.
I asked her about her problems.
I asked her whether she had problems. (More natural than ‘I asked her about whether she had problems’.)