There are quite a lot of situations that make it possible or necessary to write an infinitive verb without “to”
THE POSSIBILITY OF DROPPING “to” FROM ENGLISH INFINITIVES
English language courses usually make it clear quite early on that “infinitive” verbs (those that lack an ending after to) are sometimes used without the to. A well-known occasion, for example, is after “modal” verbs like will, can, should and must. However, detailed surveys of all the situations where so-called “bare” infinitives are possible or necessary are much harder to find. It is this kind of survey that I am attempting in the present post, in the hope of resolving every possible uncertainty that readers might have about the use of bare infinitives. I will consider first situations where dropping to is optional, and then those where it is compulsory.
A preliminary point to note is that the to of infinitive verbs is not a preposition (see 3. Multi-Use Words and 35. “to do” versus “to doing”). It normally helps to distinguish infinitives from other verbs without an ending, such as imperatives (see 128. Imperative Verbs in Formal Writing), subjunctives, and plurals in the present simple tense (see 12. Singular & Plural Verb Choices).
Guinlist posts about the ordinary use of to in infinitive verbs include 60. Purpose Sentences with “For”, 78. Infinitive versus Preposition after Nouns, 83. Active Verbs with Non-Active Meanings 2, 105. Questions with a “to” Verb and 119. BE Before a “to” Verb.
OPTIONAL BARE INFINITIVES
The to of an infinitive verb is most commonly optional when the infinitive is the second of a pair that are combined together by a suitable linking word. In many cases the linking word will be a conjunction, especially and:
(a) To paraphrase well, one has to comprehend and (TO) REMEMBER the source text.
The optionality of to before remember here is not surprising, given the way and generally in English allows repeated words to be dropped (see 36. Words Left out to Avoid Repetition and 64. Double Conjunctions). The conjunction than is another common one found with a dropped to:
(b) Children are often keener to play with their parents than (to) watch TV.
The other main kind of linking word between infinitives that makes the second to optional is the verb BE, placed in the middle so that the first infinitive is part of the subject and the second is the “complement” (for the closeness of subjects and complements, see 113. Verbs that Cannot be Passive). It is important that the first infinitive is only part of the subject and not all of it. Compare:
(c) To think is to exist.
(d) The way to learn a language is (to) live where it is spoken.
(e) What consumers tend to do is (to) compare product prices.
In (c), the first infinitive to think makes up all of the subject of the verb is, so that the second infinitive to exist cannot drop its to (for more about sentences of this kind, see 119. BE Before a “to” Verb). In (d) and (e), however, the first infinitives to learn and to do are each part of a longer subject, and the second infinitives can easily drop their to.
Situations where the to of an infinitive is optional but not linked to one used earlier are quite rare, but do exist. One kind may be illustrated as follows:
(f) WHAT plants do at night IS (to) absorb carbon dioxide.
This is a similar sentence to (e) (both are of the kind considered in the Guinlist post 145. Highlighting with “what” Sentences). However, there is no infinitive verb before is, but rather a “dummy” ordinary verb do which anticipates the later one absorb. In such cases, the later verb usually has to be in the infinitive form, with or without to.
One other place where a bare infinitive is optional but not linked to an earlier infinitive with to is after the verb HELP and its object, like this:
(g) Nicotine chewing gum can help smokers (to) quit.
COMPULSORY BARE INFINITIVES
There seem to be at least four situations where a bare infinitive must be used.
1. After Modal Verbs
This familiar kind of bare infinitive is illustrated in such combinations as will know, can go, may be, must have and should say. There is also a use after DO (does not think). A small potential for confusion is with the “semi-modals” NEED and DARE, since they sometimes require an infinitive with to and sometimes one without.
NEED and DARE are called “semi-modal” because in the question and negative forms they can (but do not have to) be used like modal verbs. If they are so used, a bare infinitive will be necessary. Compare the following:
Non-Modal Use (with to)
Negative Statement: … does not need/dare TO SAY
Question: does … need/dare TO SAY?
Modal Use (without to)
Negative Statement: … need not/dare not SAY
Question: need/dare … SAY?
In one sense, DARE and NEED in negative statements and questions allow a free choice over the use of to, since the modal and non-modal uses are both possible. However, once it has been decided to use NEED or DARE as a modal – for example by placing not after it instead of before in a negative statement, or a subject noun after it instead of before in a question – then there is no choice about dropping to from the following verb.
Sometimes the need for to to be dropped because of a modal verb even affects sentences like (e) above, where there would normally be a choice regarding to. This happens when the first infinitive follows a modal, so that it has no to (e.g. should do instead of tend to do). In such cases the second infinitive will also not be able to have to – the choice will disappear (… is compare product prices).
2. After the Object of Particular “Cause” Verbs
The Guinlist post 32. Expressing Consequences lists various “cause” verbs that can be followed by an infinitive, provided there is also an object noun in between, like this:
(h) Price increases usually CAUSE demand TO FALL.
Most verbs like cause in this example (ALLOW, ASSIST, ENABLE, LEAD etc.) require the following infinitive to have to. However, there are at least three exceptions: MAKE (= “cause by forcing”), LET (= “cause by allowing”) and HAVE (= “cause by organizing”). Either of the first two can be used instead of cause in (h), but will need to fall to drop its to (see 10. Words with Unexpected Grammar 1 and 141. Ways of Using MAKE). The way to use causative HAVE is shown in the following sentence from the post 116. Rarer Uses of HAVE:
(i) It is desirable to have young children work in groups.
3. After the Object of Perception Verbs
This usage may be illustrated as follows:
(j) On SEEING the liquid CHANGE colour, turn off the heat.
The verb see is one of perception. Others like it are FEEL, HEAR, LISTEN TO, NOTICE, OBSERVE, SMELL and WATCH. The subsequent bare infinitive (change) is not the only possible verb form, but is compulsory for expressing the particular meaning of a completed rather than ongoing action. The alternative form, for ongoing actions, is an -ing participle (see 52. Participles Placed Just after their Noun). Having -ing instead of a bare infinitive for a completed action would be a grammar error of the “invisible” kind (see 100. What is a Grammar Error?).
The verb HAVE – whether given the causative meaning illustrated in (i) or the “suffer” one shown below – also allows a following verb to have either -ing or a bare infinitive according to how complete the meaning is:
(k) Farmers can easily have pests eat(ing) their crops.
4. In Infinitival “Why” Questions
Questions based on an infinitive verb are the focus of the Guinlist post 105. Questions with a “to” Verb. They comprise at least a question word and an immediately-following infinitive verb, as in what to do(?), where to go(?) and whether to respond. Usually the infinitive verb needs to. However, the to must be dropped when the question word is why (e.g. why agree?).