Prepositions cannot normally be followed by conjunctions. Of course, this is possible in a few cases, but prepositions are normally dropped before that-clauses. This usually happens after words referring to saying, writing, thinking etc.
- I knew about his illness. (Here the preposition about is followed by a noun.)
- I knew that he was ill. (NOT I knew about that he was ill.)
- I heard about his problems.
- I heard that he was facing some problems.
- I had no idea of his state of mind.
- I had no idea that he was depressed.
- I was not aware of his presence in the room. (Here the preposition of is followed by the noun his presence.)
- I was not aware that he was present in the room. (NOT I was not aware of that he …)
In cases where the preposition cannot be dropped before that, we add the phrase ‘the fact’.
- I managed to arrive in time for the meeting in spite of the fact that the train was late.
- (NOT I managed to arrive in time for the meeting in spite of that …) (NOT I managed to arrive in time in spite that …)
- She paid no attention to the fact that I had the necessary qualifications. (NOT She paid no attention that I had the necessary qualifications.)
- They went out despite the fact that it was raining. (NOT They went out despite that it was raining.)