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 ...

In order to produce and comprehend English correctly, students need to know how to form infinitives, when to use infinitive clauses, and, most importantly, how to interpret the "logical subject" of an ...