Prepositions before conjunctions

Prepositions can be followed by conjunctions in some cases but not in others.

Indirect speech: prepositions dropped before that

Prepositions are not used directly before the conjunction that. In indirect speech, after words that refer to saying, writing, thinking, etc, prepositions are usually dropped before that-clauses. Compare:

• I knew about his problems.
  I knew that he had problems. (not I knew about that he had problems.)
• She had no idea of my state of mind.
  She had no idea that I was unhappy. (not She had no idea of that I was unhappy.)
• I wasn’t aware of the time.
  I wasn’t aware that it was so late. (not I wasn’t aware of that it was so late.)

Emotional reactions: prepositions dropped

Prepositions are also dropped before that after many common words that refer to emotional reactions. Compare:

• We are sorry about the delay.
  We are sorry that the train is late. (not … sorry about that the train is late.)
• I was surprised at her strength.
  I was surprised that she was so strong. (not … surprised at that she was …)

the fact that

In other cases (not involving indirect speech or words referring to emotional reactions) prepositions cannot so often be dropped before that-clauses. Instead, the expression the fact (see here) is generally put between the preposition and that.

• The judge paid a lot of attention to the fact that the child was unhappy at home. (not The judge paid a lot of attention (to) that the child …)
• He said the parents were responsible for the fact that the child had run away. (not … responsible (for) that the child had run away.)

Question words

After some very common words like tell, ask, depend, sure, idea, look, prepositions can be dropped before who, which, what, where, whether and other question words. This is especially common in indirect questions. Compare:

• Tell me about your trip.
  Tell me (about) where you went.
• I asked her about her religious beliefs.
  I asked her whether she believed in God. (More natural than I asked her about whether she believed in God.)
• We may be late – it depends on the traffic.
  We may be late – it depends (on) how much traffic there is.
• I’m not sure of his method.
  I’m not sure how he does it. (More natural than I’m not sure of how he does it.)
• I’ve no idea of the owner.
  I’ve no idea (of) who owns it.
• Look at this.
  Look (at) what I’ve got.

In other cases it is unusual or impossible to leave out the preposition.

• I’m worried about where she is. (not I’m worried where she is.)
• The police questioned me about what I’d seen. (not The police questioned me what I’d seen.)
• There’s the question of who’s going to pay. (More natural than … the question who’s going to pay.)
• People’s chances of getting jobs vary according to whether they live in the North or the South. (not … according whether …)

If does not normally follow prepositions; we use whether (see here) instead.

• I’m worried about whether you’re happy. (not I’m worried about if …)

note

For the structures (with and without prepositions) that are possible after a particular verb, noun or adjective, see a good dictionary.