Auxiliary verb

An auxiliary verb is a verb that adds functional or grammatical meaning to the clause in which it occurs, so as to express tense, aspect, modality, voice, emphasis, etc. Auxiliary verbs usually accompany an infinitive verb or a participle, which respectively provide the main semantic content of the clause. An example is the verb have in the sentence I have finished my lunch. Here, the auxiliary have helps to express the perfect aspect along with the participle, finished. Some sentences contain a chain of two or more auxiliary verbs. Auxiliary verbs are also called helping verbs, helper verbs, or auxiliaries. Research has been conducted into split inflection in auxiliary verbs.

Basic examples

Below are some sentences that contain representative auxiliary verbs from English, Spanish, German and French, with the auxiliary verb marked in bold:
These auxiliaries help express a question, show tense/aspect, or form passive voice. Auxiliaries like these typically appear with a full verb that carries the main semantic content of the clause.

Traits across languages

Auxiliary verbs typically help express grammatical tense, aspect, mood, and voice. They generally appear together with an infinitive. The auxiliary is said to "help" the infinitive. The auxiliary verbs of a language form a closed class, i.e., there is a fixed, relatively small number of them.
Widely acknowledged verbs that can serve as auxiliaries in English and many related languages are the equivalents of be to express passive voice, and have to express perfect aspect or past time reference.
In some treatments, the copula be is classed as an auxiliary even though it does not "help" another verb, e.g.,
Definitions of auxiliary verbs are not always consistent across languages, or even among authors discussing the same language. Modal verbs may or may not be classified as auxiliaries, depending on the language. In the case of English, verbs are often identified as auxiliaries based on their grammatical behavior, as described below. In some cases, verbs that function similarly to auxiliaries, but are not considered full members of that class, are called semi-auxiliaries. In French, for example, verbs such as devoir, pouvoir, aller, vouloir, faire, and laisser, when used together with the infinitive of another verb, can be called semi-auxiliaries. There has also been a study on auxiliary verb constructions in Dravidian languages.

English

The following sections consider auxiliary verbs in English. They list auxiliary verbs, then present the diagnostics that motivate this special class. The modal verbs are included in this class, due to their behavior with respect to these diagnostics.

List of auxiliaries in English

A list of verbs that function as auxiliaries in English is as follows:
be, can, could, dare, do, have, may, might, mote, must, need, ought, shall, should, will, would.
The status of dare , need , and ought is debatable and the use of these verbs as auxiliaries can vary across dialects of English. If the negative forms can't, don't, won't, etc. are viewed as separate verbs, then the number of auxiliaries increases. The verbs do and have can also function as full verbs or as light verbs, which can be a source of confusion about their status. The modal verb|modal verb]s form a subclass of auxiliary verbs. Modal verbs are defective insofar as they cannot be inflected, nor do they appear as gerunds, infinitives, or participles.
The following table summarizes the auxiliary verbs in standard English and the meaning contribution to the clauses in which they appear. Many auxiliary verbs are listed more than once in the table based upon discernible differences in use.

Auxiliary verb	Meaning contribution	Example
be₁	copula	She is the boss.
be₂	progressive aspect	He is sleeping.
be₃	passive voice	They were seen.
can₁	ability	I can swim.
can₂	permission	Can I come in?
can₃	suggestion or offer	I can come with you.
could₁	past tense of can₁	I could swim.
could₂	past tense of can₂	I couldn't leave.
could₃	can₁ hypothetical	I couldn't play guitar without hands.
could₄	can₂ hypothetical	I could participate if I was over 18.
could₅	can₃ hypothetical	I could stay here in case he comes back.
could₆	can₁ uncertain	That could help.
could₇	can₂ uncertain or hesitant	Could I come in?
could₈	can₃ uncertain or hesitant	I could say something if you want me to.
dare	deontic modality	I dare not attempt it.
do₁	do-support for negation	You did not understand.
do₂	emphatic do	I do understand.
do₃	question-forming do-support	Do you understand?
have	perfect aspect	They have understood.
may₁	permission	May I stay?
may₂	possibility	That may happen.
might₁	possibility	That might happen.
might₂	humble request	Might I ask for your advice?
mote	optative mood	So mote it be.
must₁	obligation	You must not mock me.
must₂	logical deduction	It must have rained.
need	deontic modality	You need not water the grass.
ought₁	exhortation or judgment	You ought to stay home after dark.
ought₂	expectation	You ought to play well after a good night's sleep.
shall₁	insistence or obligation	The renter shall be responsible for the damage.
shall₂	permission	You shall enter.
should₁	advice	You should listen.
should₂	expectation	That should help.
should₃	hypothetical	If I should fail, please help me.
should have	past tense of shall	The landlord should have paid for it.
will₁	future tense	The sun will rise tomorrow at 6:03.
will₂	habitual aspect	He will make that mistake every time.
will₃	predictive inference	He will be home by now.
will₄	ability	Only one key will fit.
will₅	intention or request	I will have a Caesar salad.
would₁	past tense of will₁	After 1990, we would do that again.
would₂	past tense of will₂	Back then we would always go there.
would₃	past tense of will₃	He would be home.
would₄	past tense of will₄	Only one key would fit.
would₅	past tense of will₅	We would have pie.
would₆	hypothetical of will₁	I would go tomorrow.
would₇	hypothetical of will₂	He would make that mistake every time.
would₈	hypothetical of will₃	He would be home by now.
would₉	hypothetical of will₄	The shirt would fit if I lost weight.
would have	past tense hypothetical of will	He would have gone home.

Deontic modality expresses an ability, necessity, or obligation that is associated with an agent subject. Epistemic modality expresses the speaker's assessment of reality or likelihood of reality. Distinguishing between the two types of modality can be difficult, since many sentences contain a modal verb that allows both interpretations.

List of auxiliaries unique to African American Vernacular English

makes a variety of finer tense/aspect distinctions than other dialects of English by making use of unique variant forms of, in particular: habitual 'be', reduced 'done', and stressed 'been' :

Auxiliary	Meaning	Example
be	habitual aspect	She be telling people she eight. 'She is always telling people she's eight'
done	resultative modality	I done pushed it. 'I have pushed it'
been	distant past tense	I been knew that. 'I've known that for a long time'

Diagnostics for identifying auxiliary verbs in English

The verbs listed in the previous section can be classified as auxiliaries based upon two diagnostics: they allow subject–auxiliary inversion and they can take not as a postdependent. The following examples illustrate the extent to which subject–auxiliary inversion can occur with an auxiliary verb but not with a full verb..
The following examples illustrate that the negation not can appear as a postdependent of a finite auxiliary verb, but not as a postdependent of a finite full verb:
A third diagnostic that can be used for identifying auxiliary verbs is verb phrase ellipsis. See the article on verb phrase ellipsis for examples.
These criteria lead to the copula be and non-copular use of be as an existential verb being considered an auxiliary. However, if one defines auxiliary verb as a verb that somehow "helps" another verb, then the copula be is not an auxiliary, because it appears without another verb. The literature on auxiliary verbs is somewhat inconsistent in this area.
There are also some properties that some but not all auxiliary verbs have. Their presence can be used to conclude that the verb is an auxiliary, but their absence does not guarantee the converse. One such property is to have the same form in the present tense, also for the first and the third person singular. This in particular is typical for modal auxiliary verbs, such as will and must.

Light verbs

Some syntacticians distinguish between auxiliary verbs and light verbs. The two are similar insofar as both verb types contribute mainly just functional information to the clauses in which they appear. Hence both do not qualify as separate predicates, but rather they form part of a predicate with another expression – usually with a full verb in the case of auxiliary verbs and usually with a noun in the case of light verbs.
In English, light verbs differ from auxiliary verbs in that they cannot undergo inversion and they cannot take not as a postdependent. The verbs have and do can function as auxiliary verbs or as light verbs. When they are light verbs, they fail the inversion and negation diagnostics for auxiliaries, e.g.
Note that in some dialects, the inversion test may sound correct to native speakers.
Sometimes the distinction between auxiliary verbs and light verbs is overlooked or confused. Certain verbs may be judged as light verbs by some authors, but as auxiliaries by others.