When do we use afraid + infinitive (to climb, to say, to go, etc.) and when do we use afraid of + …ing form (climbing, saying, going, etc.)?
We use afraid + infinitive when we explain the fear itself:
- I am afraid to go out in the dark.
- He is afraid to travel by plane.
- They are afraid to cross the high bridge over the river.
We use afraid of …ing when we see an unwelcome result of an action:
- I am afraid of falling. (= I am afraid because I might fall.)
- He is afraid of making a mistake. (= He is afraid because he may make a mistake.)
- They are afraid of getting lost in the forest. (= They are afraid because they may get lost.)
Taken from Oxford Grammar Tips