In linguistics, an auxiliary (also called helping verb, helper verb, auxiliary verb, or verbal auxiliary) is a verb functioning to give further semantic or syntactic information about the main or full verb following it. In English, the extra meaning an auxiliary verb imparts alters the basic form of the main verb to have one or more of the following functions: passive, progressive, perfect, modal, or dummy. In English, every clause has a finite verb which consists of a full verb (a non-auxiliary verb) and optionally one or more auxiliary verbs, each of which is a separate word. Examples of finite verbs include write (no auxiliary verb), have written (one auxiliary verb), and have been written (two auxiliary verbs). There is a syntactic difference between an auxiliary verb and a full verb; that is, each has a different grammatical function within the sentence. In English, and in many other languages, there are some verbs that can act either as auxiliary or as full verbs, such as be ("I am writing a letter" vs "I am a postman") and have ("I have written a letter" vs "I have a letter"). In the case of be, it is sometimes ambiguous whether it is auxiliary or not; for example, "The ice cream was melted" could mean either "Someone/something melted the ice cream" (in which case melt would be the main verb) or "the ice cream was mostly liquid" (in which case be would be the main verb). The auxiliary verb be is used with a past participle to form the passive voice; for example, the clause "the door was opened" implies that someone (or something) opened it, without stating who (or what) it was. Because many past participles are also stative adjectives, the passive voice can sometimes be ambiguous; for example, "at 8:25, the window was closed" can be a passive-voice sentence meaning "at 8:25, someone closed the window," or a non-passive-voice sentence meaning "at 8:25, the window was not open". Perhaps due to this ambiguity, the verb get will sometimes be used colloquially instead of be in forming the passive voice, "at 8:25, the window got closed". The auxiliary verb be is used with a present participle to form the progressive aspect; for example, the sentence "I am riding my bicycle" describes what the speaker is doing at the very moment of utterance, while the sentence "I ride my bicycle" is a temporally broader statement. The auxiliary verb have is used with a past participle to form the perfect aspect; for example, the sentence "Peter has fallen in love" differs from "Peter fell in love" in that the former implies some connection to the present — likely that Peter is still in love — while the latter does not. There are nine modal verbs: can, could, may, might, shall, should, will, would, and must. They differ from the other auxiliaries both in that they are defective verbs, and in that they can never function as main verbs. (There do exist main verbs can and will, but these are distinct.) They express the speaker's (or listener's) judgement or opinion at the moment of speaking. Some of the modal verbs have been seen as a conditional tense form in English. In American English, go and come can be quasi-auxiliaries with nothing between them and the following verb phrases, but only in their plain forms: "Come show me", "I'll go get it", and "I had to come see for myself". This use can be regarded as ellipsis of and — the previous are equivalent to "Come and show me", "I'll go and get it", and "I had to come and see for myself" — and British English requires the and to be included, as does American English when the verb is not in its plain form: "I went and saw him." (It is also possible in both dialects for to to be used in place of and, though this typically has a slightly different sense.) Auxiliaries take not (or n't) to form the negative, e.g. can't, won't, shouldn't, etc. In certain tenses, in questions, when a contracted auxiliary verb can be used, the position of the negative particle n't moves from the main verb to the auxiliary: cf. Does it not work? and Doesn't it work? Auxiliaries can be repeated at the end of a sentence, with negation added or removed, to form a tag question. In the event that the sentence did not use an auxiliary verb, a dummy auxiliary (a form of do) is used instead:
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