A practical english grammar - Vyssaja skola 1978
Question words with infinitives
Sound symbols
A construction consisting of a question word and an infinitive often functions as a noun equivalent. It can usually be construed as a condensed type of noun clause, the implied subject of the infinitive being made clear by the context. If a noun follows the question word, or if the question word is who (whom in formal style), the noun or question word is the object of the infinitive.
I don’t know where to go. (= where I should go)
I can’t decide whom to invite. (= whom I should invite)
He asked me how to open (= how he should or could the box. open the box)
This construction occurs as an object after a limited number of characteristic verbs, such as, know, ask, learn, teach, tell (someone), say, find out, and others of similar meaning.
Note that the action expressed by the infinitive is always potential rather than actual. This construction does not narrate events; it names events that may (will, should) occur in the future.
The use of the construction in other parts of the sentence (as subject, complement, object of preposition) is shown below:
How to refuse the invitation without offending them was the big question.
The main problem was what to do next.
The lecturer talked about how to raise rabbits.
His question had to do with which problem to attack first.