Regarding English, the term infinitive is traditionally applied to the unmarked form of the verb 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.
Moreover, the unmarked form of the verb is not considered an infinitive when it is 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."
There are nonfinite constructions that are marked for perfect, progressive or perfect progressive aspect, using the infinitives, participles or gerunds of the appropriate auxiliaries. The meanings are as would be expected for the respective aspects: perfect for prior occurrence, progressive for ongoing occurrence at a particular time. (Passive voice can also be marked in nonfinite constructions – with infinitives, gerunds and present participles – in the expected way: (to) be eaten, being eaten, having been eaten, etc.)
Examples of nonfinite constructions marked for the various aspects are given below.
- You should have left earlier. (perfect infinitive; for similar constructions and their meanings see modal verbs)
- She might be revising. (progressive; refers to an ongoing action at this moment)
He must have been working hard. (perfect progressive; i.e. I assume he has been working hard)
- He is said to have resigned. (perfect infinitive)
- I expect to be sitting here this time tomorrow. (progressive)
- He claims to have been working here for ten weeks. (perfect progressive)
- Having written the letter, she went to bed. (perfect)
- The man having left, we began to talk. (perfect, in a nominative absolute construction)
- Having been standing for several hours, they were beginning to feel tired. (perfect progressive)
- We have been waiting a long time. (progressive, used only as part of a perfect progressive construction)
- My having caught the spider impressed the others. (perfect)
- We are not proud of having been drinking all night. (perfect progressive)
Other aspectual, temporal and modal information can be marked on nonfinite verbs using periphrastic constructions. For example, a "future infinitive" can be constructed using forms such as (to) be going to eat or (to) be about to eat.