[[English language|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 [[grammatical particle|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 there is a controversy about whether it should be separated from the main word of the infinitive. (''See'' [[Split infinitive]].) Nonetheless, modern theories typically do not consider the to-infinitive to be a distinct [[constituent (linguistics)|constituent]], instead taking the particle ''to'' to operate on an entire verb phrase; so, ''to buy a car'' is parsed as ''to [buy [a car]]'', not as ''[to buy] [a car]''.
The bare infinitive and the full infinitive are mostly in [[complementary distribution]]. They 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 full infinitive.
[[Rodney Huddleston|Huddleston]] and [[Geoffrey Pullum|Pullum]]'s recent{{When|date=April 2010}} ''Cambridge Grammar of the English Language'' (CGEL) does not use the notion of the ''infinitive'', arguing that English uses the same form of the verb, the ''plain form'', in infinitival clauses that it uses in [[Imperative mood|imperative]] and present-[[subjunctive]] clauses.
===Uses of the bare infinitive===
The bare infinitive is not used in as many contexts as the full infinitive, but some of these are quite common:
*The bare infinitive is used as the [[main verb]] after the dummy auxiliary verb ''do'', or most [[modal auxiliary verb]]s (such as ''will'', ''can'', or ''should''). 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.")
*After the ''had better'' expression. So, "You had better '''leave''' now."
*With the verb ''help''. So, "He helped them '''find''' it."
*With the word ''why''. So, "Why '''reveal''' 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 mood|subjunctive]] form and the [[imperative mood|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:
*Outside of dictionary headwords, it is the most commonly used [[citation form]] of the English verb: "How do we conjugate the verb ''to go''?"
*It can be used like a noun phrase, expressing its action or state in an abstract, general way. So, "'''To err''' is human"; "'''To know''' me is '''to love me'''". (However, a [[gerund]] is often preferred for this — "'''Being''' is '''doing'''" would be more natural than the abstract and philosophical sounding "'''To be''' is '''to do'''."
[[http://englishpage.com/gerunds/part_1.htm English Page - Gerunds and Infinitives Part 1] [sic]])
*It can be used like an adjective or adverb, expressing purpose or intent. So, "The letter says I'm '''to wait outside'''", or "He is the man '''to talk to'''", 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 [[Control (linguistics)|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-like use above.) With some verbs the infinitive may carry a significantly different meaning from a gerund: compare ''I stopped to talk to her'' with ''I stopped talking to her'', or ''I forgot to buy the bread'' with ''I forgot buying the bread''.
*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." However, in some cases, the subject of the main clause is also subject of the infinitival clause, as in "John promises Mary to cook", where the cook is John (the subject of the main sentence), and not Mary (the object).
*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 verb]]s, ''can'', ''may'', ''shall'', ''will'' and ''must'' are [[defective verb|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 [[compound verb|periphrases]] ''to be able to'', ''to have to'' and ''to be going to'' are generally used in these cases.
=== Impersonal constructions ===
There is a specific situation in which the infinitive is used like an "impersonal future tense", replacing "will". This is done through the construction:
::''to be'' + "''to''" + bare infinitive
Grammatically, this is identical to the instructional "I am to wait outside" construction (above), but ''does not'' signify somebody having been issued an instruction; rather, it expresses an intended action, in the same way as "will". This "tense" is used extensively in news reports, eg. –
* ''The Prime Minister is to visit the West Bank'' (active)
* ''Aid is to be sent to war-torn Darfur'' (passive) '''
[In headlines, the verb ''to be'' is entirely omitted - eg. ''Prime Minister to visit...; Aid to be sent...,'' etc.]'''
This "future infinitive" construction is interesting in that it only has a future aspect to it in situations where the speaker is significantly distanced from the event.
[Grammar books on English simply do not deal with this tense due to its extreme rarity, hence why it has no official name {{Citation needed|date=June 2008}}.] In cases where the subject of the sentence is not quite as distanced from the speaker, then the same construction takes on a sense of '''''instruction''''' or '''''necessity''''' (as in "he is to wait outside", or "he is to go to hospital").
The same construction can be used in conditional clauses - ''If you are to go on holiday, then you need to work hard'' (or, conversely, ''if you want to...then you are to...'').
The impersonality aspect comes from the fact that the emotionless verb ''to be'' is used in the place of the more usual [[modal verbs]] which would normally connect the speaker to the statement. In this way, statements are given weight (as if some external force, rather than the speaker, is governing events).
Conversely, however, the construction also provides an uncertainty aspect, since it frees the speaker from responsibility on their statement – in the phrase "John will go", for example, the speaker is almost advocating their certainty that John will, in fact, go; meanwhile, "the Prime Minister is to go" simply states the '''''knowledge''''' that the PM's going is in some way foreseen. (If John ends up not going, for example, the "will go" construction is negated, while the PM's "to go" construction would still hold true, since all it expresses is an ''expectation''). In both cases, the knowledge is simply being reported (or pretends to be) from an independent source. In this sense, this impersonal ''to + verb'' construction can almost be seen as a fledgeling [[Grammatical mood#Inferential or renarrative|renarrative]] mood.
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