Infinitives in English

English has three non-finite verbal forms, but by long-standing convention, the term "infinitive" is applied to only one of these. (The other two are the past- and present-participle forms, where the present-participle form is also the gerund form.) In English, a verb's infinitive is its unmarked form, such as "be," "do," "have," or "sit," often introduced by the particle "to." When this particle is absent, the infinitive is said to be a bare infinitive; when it is present, it is generally considered to be a part of the infinitive, then known as the full infinitive (or to-infinitive), and some grammarians hold that it should not be separated from the main word of the infinitive. (See split infinitive.)

The bare infinitive and the full infinitive are not generally interchangeable, but the distinction does not generally affect the meaning of a sentence; rather, certain contexts call almost exclusively for the bare infinitive, and all other contexts call for the to-infinitive.

Uses of the bare infinitive

The bare infinitive is used in a rather limited number of contexts, but some of these are quite common:

• The bare infinitive is used as the main verb after the dummy auxiliary verb do, or any modal auxiliary verb (such as will, can, or should), except that ought usually takes a to-infinitive. So, "I will/do/can/etc. see it."
• Several common verbs of perception, including see, watch, hear, feel, and sense take a direct object and a bare infinitive, where the bare infinitive indicates an action taken by the main verb's direct object. So, "I saw/watched/heard/etc. it happen." (A similar meaning can be effected by using the present participle instead: "I saw/watched/heard/etc. it happening." The difference is that the former implies that the entirety of the event was perceived, while the latter implies that part of the progress of the event was perceived.)
• Similarly with several common verbs of permission or causation, including make, bid, let, and have. So, "I made/bade/let/had him do it." (However, make takes a to-infinitive in the passive voice: "I was made to do it.")
• The bare infinitive is the dictionary form of a verb, and is generally the form of a verb that receives a definition; however, the definition itself generally uses a to-infinitive. So, "The word 'amble' means 'to walk slowly.'"
• The bare infinitive form is also the present subjunctive form and the imperative form, although most grammarians do not consider uses of the present subjunctive or imperative to be uses of the bare infinitive.

Uses of the full infinitive

The full infinitive (or to-infinitive) is used in a great many different contexts:

• Apart from in dictionary lemmata, the full infinitive is the most commonly used citation form of the English verb: "How do we conjugate the verb to go?"
• It can serve as an ordinary noun, expressing its action or state in an abstract, general way. So, "To be is to do." (A gerund can also be used for this: "Being is doing.")
• It can serve as an adjective or adverb, expressing purpose or intent. So, "He is [supposed] to die at noon," or "[In order] to meditate, one must free one's mind."
• In either of the above uses, it can often be given a subject using the preposition for: "For him to fail now would be a great disappointment"; "[In order] for you to get there on time, you'll need to leave now." (The former sentence could also be written, "His failing now would be a great disappointment.")
• It can be used after many intransitive verbs; in this case, it generally has the subject of the main verb as its implicit subject. So, "I agreed to leave," or "He failed to make his case." (This may be considered a special case of the noun use above.)
• It can be used after the direct objects of many transitive verbs; in this case, it generally has the direct object of the main verb as its implicit subject. So, "I convinced him to leave with me," or "He asked her to make his case on his behalf."
• As a special case of the above, it can often be used after an intransitive verb, together with a subject using the preposition for: "I arranged for him to accompany me," or "I waited for summer to arrive."

When the verb is implied, some dialects will reduce the to-infinitive to simply to: "Do I have to?"

The infinitive with auxiliary verbs

The auxiliary verb do does not have an infinitive - even though do is also a main verb and in that sense is often used in the infinitive. One does not say *I asked to do not have to, but rather, either I asked not to have to or I asked to not have to (but see split infinitive). Similarly, one cannot emphasize an infinitive using do; one cannot say, "I hear him do say it all the time."

Nonetheless, the auxiliary verbs have (used to form the perfect aspect) and be (used to form the passive voice and continuous aspect) both commonly appear in the infinitive: "It's thought to have been a ceremonial site," or "I want to be doing it already."

Defective verbs

The modal auxiliary verbs, can, may, shall, will and must are defective in that they do not have infinitives; so, one cannot say, *I want him to can do it, but rather must say, I want him to be able to do it. The circumlocutions to be able to, to have to and to be going to are generally used in these cases.