The following English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 verb

Verb

A verb, from the Latin verbum meaning word, is a word that in syntax conveys an action , or a state of being . In the usual description of English, the basic form, with or without the particle to, is the infinitive...

 forms can appear as auxiliary verb

Auxiliary verb

In linguistics, an auxiliary verb is a verb that gives further semantic or syntactic information about a main or full verb. In English, the extra meaning provided by an auxiliary verb alters the basic meaning of the main verb to make it have one or more of the following functions: passive voice,...

s. Note that some of these forms can also be used as main verbs. The main criterion used here for whether something is an auxiliary verb is whether it participates in subject-auxiliary inversion

Subject-auxiliary inversion

In English, subject-auxiliary inversion occurs when an auxiliary verb precedes a subject. This is an exception to the English word order convention of subjects preceding their corresponding verbs...

.

• am, aren't (only in inversion, as in Aren't I running fast?), ain't
  Ain't
  Ain't is a colloquialism and contraction for "am not", "is not", "are not", "has not", and "have not" in the common English language vernacular. In some dialects ain't is also used as a contraction of "do not", "does not", and "did not". The usage of ain't is a perennial subject of controversy in...
  
  , 'm (as in I'm)
• are, aren't, ain't, 're (as in you're)
• be (NB: this is an infinitive/imperative form and as such does not participate in subject inversion or tag questions)
• been
• better, had better, 'd better (as in You'd better go now)
• can, can't
• could, couldn't
• dare
• did, didn't
• do, don't
• does, doesn't
• had, hadn't, 'd (as in She'd gone out)
• has, hasn't, 's (as in She's gone out)
• have, haven't, 've (as in I've)
• is, isn't, 's (as in She's back)
• keep, (as in Keep writing)
• may, mayn't
• might, mightn't
• must, mustn't
• need, needn't
• ought (to), oughtn't (to)
• shall, shan't
• should, shouldn't
• used (to)
• was, wasn't
• were, weren't
• will, won't, 'll (as in she'll)
• would, wouldn't, 'd (as in I'd go out)

The contracted forms can be stacked, e.g. I'd've told her to leave, or She'll've left already by the time you get there.

Dare, need, used (to), and ought (to) are sometimes considered auxiliaries, but they do not permit subject-auxiliary inversion in many dialects. When they are auxiliaries, they permit sentences such as Dare you go? (with non-auxiliary equivalent Would you dare to go?), Need you say this? (Do you need to say this?), Used we to go?, and Ought we to go through with this? ((Do we have to / Should we) go through with this?). When dare and need are used as auxiliaries, that use is always in either the interrogative or the negative (He dare not go; He need not go), neither the auxiliary nor the main verb conjugates for the third person singular, and the particle to is not included before the bare infinitive. However, Warner rejects ought and used as auxiliaries because the subsequent infinitive form includes the particle to.

Palmer gives arguments for also including better or 'd better (or had better) as auxiliaries (as in the unconjugated He had better go, He had better not go, Had he better not go?). But Warner does not include it.