A question word with a “to” verb expresses a special meaning and is quite common in formal writing
THE OCCURRENCE OF 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 structure that most learners of English encounter first, where the verb after the question word must have a subject. Even indirect questions can be written with a to verb, their indirect status being clear from clues outside them, such as the absence of a question mark (see 57. Indirect Questions in Formal Writing) rather than their own form:
(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 indirect one above can be rewritten …how they can avoid infection, and the one in the cartoon means Who can 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. A further question is where questions with a to verb are likely to be found, assuming one is possible.
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QUESTIONS THAT CAN BE ASKED WITH A “to” VERB
Direct questions seem less generally usable with a to verb than indirect ones. Those where a to verb is possible seem mainly to be not information-seeking but indicating the speaker’s inability to find an answer. In the cartoon above, for example, the speaker is expressing an inability to identify trustworthy people, thereby implying that none exist. As another example, Where to go? would indicate uncertainty about a destination.
Indirect questions, by contrast, often allow a to verb where a direct one would not. This is the case, for example, with the common information-seeking question quoted by a reader in the comment section after this post:
(c) Can you tell me how to say X in English?
One major characteristic of all questions with a to verb is that they typically begin with a question word. However, there are exceptions, and there are some question words that cannot make a to question.
The major type of question that is ruled out as makeable with a to verb because it lacks a starting question word is direct yes/no questions – the kind that start with a verb rather than a question word – like this:
(d) Should we resign our membership?
On the other hand, indirect yes/no questions, which typically begin with the question word whether or if, are able to be made with a to verb, provided they have whether rather than if (e.g. wondered whether to... – see 99. Meanings of “whether…or…”).
The main kind of question without a question word that can be made with to paraphrases the question word with a noun. It can only be indirect (see 285. Complexities of Question Words, #2). Every question word has at least one corresponding noun. For example, who corresponds to person, why to reason and how to way (see 185. Noun Synonyms of Question Words). Thus, the indirect question how to avoid in sentence (a) above could be paraphrased as the way to avoid.
Four mainstream question words that are sometimes grammatically unable to be used in a to question are what, which, who and whose. They can accompany a to verb only when making all or part of the object of the verb after them, like who in the cartoon above. When they are the subject, like who before lives in sentence (b) above, the question must be of the standard kind.
A second major characteristic of questions with a to verb is that the corresponding ordinary-form question is likely to be one with can or should. The question in the cartoon implies either of these; the question in sentence (a) suggests can; indirect to questions with whether usually seem to imply should, as does the following:
(e) Nervousness can make a speaker forget what to say (= what s/he should say).
Another special implication of questions with a to verb is who that verb’s subject is. 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 (e) is a speaker. In general, direct questions with to must usually imply either I, we, one or you (= “one”); while indirect ones must usually imply either one or the same subject as that of the verb accompanying them (indirect questions nearly always having an accompanying verb – see 219. Wording next to Indirect Questions).
Here is an indirect question whose to verb implies the subject one, meaning “people in general”:
(f) Customers ask how to prevent leakage (= how one can prevent leakage).
The implication of the subject nurses in (a) and a speaker in (d) shows how a to verb in an indirect question can have the same implied subject as that of the verb outside the question.
If the subject of the verb in an indirect question is not either one or a repetition of the subject of the external verb, the question cannot usually be asked with to. This is the case in the following examples (subjects underlined):
(g) The question IS how spaceships CAN REACH the stars.
(h) Researchers WONDER what governments CAN DO about global warming.
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OTHER FEATURES OF “to” QUESTIONS
Questions introduced by why, both direct and indirect, can be asked with a to verb, but they must drop the to! For example, one must say why work hard(?) rather than *why to work hard(?) (see 148. Infinitive Verbs without “to”, #5 and 274. Questions with a Hidden Meaning, #3).
Indirect to questions generally behave in the same way as ordinary indirect questions. Their ability to replace the question word with a noun has already been mentioned. Further examples are:
(i) Caesar understood the way (= HOW) to defeat his enemies.
(j) One must establish the distance (= HOW FAR) to travel in a day.
Note, however, that reason, the noun for why, allows a choice about putting its following verb into the to form:
(k) This essay will investigate reasons for providing (to provide) health care.
Another similarity that indirect to questions share with ordinary indirect questions is their ability to have a preposition with the question word, either before or after (see 285. Complexities of Question Words, #6):
(l) Columbus was unsure by which route to return (… which route to return by).
(m) The people no longer knew in whom to place their trust (… who to place their trust in).
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“to” QUESTIONS IN FORMAL WRITING
In formal writing, it is quite acceptable, when a question can be asked with a to verb, to use one. One of the benefits is conciseness, since to questions need fewer words than their alternative. Indirect to questions will normally be more common than direct ones because formal writing is a context where the meaning of the latter is rarely appropriate.
The main formal uses of indirect to questions will be the same as those of ordinary indirect questions: reporting, topic-introducing or ignorance-asserting (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.
To questions with a topic-introducing purpose seem especially common in official information leaflets. One reason may be that this kind of writing is often advice-giving, where many of the verbs include the meaning of can or should just as to questions do (see 187. Advising and Recommending). Using to instead of can or should is perhaps preferred because it is more friendly-sounding (see 166. Appropriacy in Professional English).
Within information leaflets, topic-introducing to questions are very likely to appear as section headings – replacing a noun that is more typical of headings elsewhere (see 178. How to Write a Heading). Common examples are Where to Go, What to Do Next and How to Apply. Such headings tend to be indirect to questions, with no question mark after them.
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