The 'remember' group
These verbs are concerned with knowing, remembering, forgetting and understanding what to do or how to do something. With most of the verbs in this group the to-infinitive is most frequently introduced by how or what, but in the case of know, it is sometimes also introduced by whether.
forget know realize recall remember see think understand
I've forgotten what to say.
Kemp didn't know whether to believe her or not.
I just couldn't remember how to spell the most simple of words.
I couldn't see how to make money.