He wants to know when to use the gerund (doing) and when to use the infinitive (to do). 1. After a preposition, we use the gerund. The prepositions can come after an adjective, after a verb or at ...

For example, when you use the verb need in a sentence, you can use an infinitive clause after need, as in the first sentence below, but you will never use a finite clause as in the second sentence.