Infinitive Phrases and clauses Uses of the infinitive

(to) eat (plain infinitive, active)

(to) be eaten (passive)

(to) have eaten (perfect active)

(to) have been eaten (perfect passive)

(to) be eating (progressive active)

(to) be being eaten (progressive passive)

(to) have been eating (perfect progressive active)

(to) have been being eaten (perfect progressive passive, not often used)

Further constructions can be made with other auxiliary-like expressions, like (to) be going to eat or (to) be about to eat, which have future meaning. For more examples of the above types of construction, see Uses of English verb forms § Perfect and progressive non-finite constructions.

Perfect infinitives are also found in other European languages that have perfect forms with auxiliaries similarly to English. For example, avoir mangé means "(to) have eaten" in French.

English

Regarding English, the term "infinitive" is traditionally applied to the unmarked form of the verb (the "plain form") when it forms a non-finite verb, whether or not introduced by the particle to. Hence sit and to sit, as used in the following sentences, would each be considered an infinitive:

• 
  I can sit here all day.
  
• 
  I want to sit on the other chair.
  

The form without to is called the bare infinitive; the form introduced by to is called the full infinitive or to-infinitive.

The other non-finite verb forms in English are the gerund or present participle (the -ing form), and the past participle – these are not considered infinitives. Moreover, the unmarked form of the verb is not considered an infinitive when it forms a finite verb: like a present indicative ("I sit every day"), subjunctive ("I suggest that he sit"), or imperative ("Sit down!"). (For some irregular verbs the form of the infinitive coincides additionally with that of the past tense and/or past participle, like in the case of put.)

Certain auxiliary verbs are defective in that they do not have infinitives (or any other non-finite forms). This applies to the modal verbs (can, must, etc.), as well as certain related auxiliaries like the had of had better and the used of used to. (Periphrases can be employed instead in some cases, like (to) be able to for can, and (to) have to for must.) It also applies to the auxiliary do, like used in questions, negatives and emphasis like described under do-support. (Infinitives are negated by simply preceding them with not. Of course the verb do when forming a main verb can appear in the infinitive.) However, the auxiliary verbs have (used to form the perfect) and be (used to form the passive voice and continuous aspect) both commonly appear in the infinitive: "I should have finished by now"; "It's thought to have been a burial site"; "Let him be released"; "I hope to be working tomorrow."

Huddleston and Pullum's Cambridge Grammar of the English Language (2002) does not use the notion of the "infinitive" ("there is no form in the English verb paradigm called 'the infinitive'"), only that of the infinitival clause, noting that English uses the same form of the verb, the plain form, in infinitival clauses that it uses in imperative and present-subjunctive clauses.^[2]

A matter of controversy among prescriptive grammarians and style writers has been the appropriateness of separating the two words of the to-infinitive (as in "I expect to happily sit here"). For details of this, see split infinitive. Opposing linguistic theories typically do not consider the to-infinitive a distinct constituent, instead regarding the scope of the particle to as an entire verb phrase; thus, to buy a car is parsed like to [buy [a car]], not like [to buy] [a car].