A combination of a question word and a “to” verb expresses a special meaning and is quite easily found in formal writing
THE POSSIBILITY OF ASKING QUESTIONS WITH A “to” VERB
As the cartoon above shows, questions can be asked with just a question word and a verb with to. This is rather different from the standard question form that most learners of English encounter first, where a question word is followed by an ordinary verb form (often of the “auxiliary” kind), along with its subject. Even indirect questions can be written with a to verb, in which case they have virtually the same form as their direct equivalent – unlike when to is absent (see 57. Indirect Questions in Formal Writing). Here is an example:
(a) Nurses must learn how to avoid infection.
Most questions with a to verb can also be phrased without one in the normal way. For example, the one in the cartoon means Who should I trust? On the other hand, many questions without a to verb cannot be rephrased with one. This is true, for example, of the following:
(b) Who lives in the Arctic Circle?
The first matter that I wish to investigate in this post is what kind of question can be asked with a to verb – fundamentally a grammar question. A further question is where questions with a to verb are likely to be found, assuming one is grammatically possible.
QUESTIONS THAT CAN BE ASKED WITH A “to” VERB
To identify when a question can be asked with to, a useful approach is to consider the way it would be asked without to. Firstly, there must still be a question word, such as who, why or how. By itself, this does not rule out any questions of an indirect kind, since they too must always have a question word (see 57. Indirect Questions in Formal Writing); but it does mean that direct questions of the “yes/no” kind, which normally begin with a verb, cannot be asked with to. This can be seen with the following:
(c) Should we resign our membership?
Secondly, questions with to seem to imply a particular kind of “modal” verb in their alternative phrasing– mostly can, should or ought to. The question in the cartoon could be rephrased with any of these, the one in sentence (a) suggests can, whereas the following indirect to question seems most equivalent to one with should:
(d) Nervousness can make a speaker forget what to say (= forget what s/he should say).
The meaning of such modal verbs tends to be either “able” (“who am I able to trust”) or “possible” (“how it is possible for them to avoid infection”) or “necessary” (“what it is necessary for him/her to say”) or “advisable”.
Another special implication of questions with to is the subject of the verb. The implied subject of trust in the question in the cartoon is I, that of avoid in (a) is nurses, and that of say in (d) is a speaker. In general, direct questions with to seem to imply either I, we, you or one, while indirect ones usually imply the same subject as that of the verb accompanying them.
The indirect question in (a), for example, implies the same subject nurses as that of the external verb must learn, while in (d) the implied indirect question subject a speaker is also the subject of forget. If the subject of a question verb is not the same as an external one, the question cannot usually be expressed with to. This is the case in the following examples (subjects underlined):
(e) The question IS whether the stars CAN BE REACHED.
(f) Researchers WONDER who CAN LIVE in extreme conditions.
Note, however, that a to verb can be used with a different subject if that subject is one, meaning “people in general”, as in this example:
(g) A common question IS how one CAN PREVENT leakage (= how to prevent leakage).
For another situation where implied subjects determine a grammar choice, see 103. Using “it” for a Subsequent Verb.
VARIATIONS WITHIN “to” QUESTIONS
Questions with a to verb have a number of noteworthy grammatical features besides their basic composition of a question word + infinitive. Firstly, they cannot have if instead of whether. If is an alternative to whether in ordinary indirect questions, like this:
(h) Caesar did not know whether/if he should return to Rome.
To rewrite the underlined question with a to verb, we have to say whether to return, not *if to return.
Secondly, an infinitive after why cannot have to: one must say why work hard rather than *why to work hard. Dropping this kind of to is a feature of some other areas of English as well; for details, see 148. Infinitive Verbs without “to”.
In other grammatical respects, indirect questions with to behave just like ordinary indirect questions. This means, firstly, that some can add a preposition to the question word, either before or after, like this:
(i) Columbus was unsure by which route to return (… which route to return by).
(j) The people no longer knew in whom to place their trust (… who to place their trust in).
In addition, to questions can replace their question word with a noun expression in the way outlined in the post 57. Indirect Questions in Formal Writing:
(k) Caesar understood the way to defeat his enemies.
(l) One must establish the distance to travel in a day.
Note, however, that reason, the noun for why, does not necessarily go with a to verb in indirect questions:
(m) This essay will investigate reasons for providing (to provide) health care.
“to” QUESTIONS IN FORMAL WRITING
In formal writing, it is quite normal, when a question can be asked with a to verb, to use this form. One of the main benefits is conciseness, since to questions need fewer words than their alternative. Indirect to questions will normally be more common than direct ones because of the general preference of formal writing for this kind of question (see 57. Indirect Questions in Formal Writing). The uses will be the same as those of ordinary indirect questions: reporting, topic-introducing or speech characterising (see 127. When to Use Indirect Speech). Here is an example of the reporting use in academic writing:
(n) Smith (2014, p. 39) wonders which factor to investigate first.
One particularly noticeable place where written to questions are found is official information leaflets. Because this kind of writing is often advice-giving, many of the verbs need to include the meaning of can, should or ought to – one of the basic features of to questions. Using to instead of these verbs is perhaps more friendly-sounding (see 166. Appropriacy in Professional English).
To questions of this kind particularly tend to appear as section headings (with the “topic-introducing” purpose). Typical examples are Where to Go, What to Do Next and How to Apply. When indirect to questions are in this form, there is only one clue that they are indirect and not direct: the absence of a question mark.