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.
Auxiliary verbs typically help express grammatical tense, aspect, mood, and voice. They typically appear together with a main verb. The auxiliary is said to "help" the main verb. The auxiliary verbs of a language form a closed class, i.e., there is a fixed, relatively small number of them. They are often among the most frequently occurring verbs in a language.
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 (and sometimes be) 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 (perhaps because they carry some independent lexical information), are called semi-auxiliaries. In French, for example, verbs such as devoir (have to), pouvoir (be able to), aller (be going to), vouloir (want), faire (make), and laisser (let), when used together with the infinitive of another verb, can be called semi-auxiliaries.
The following sections consider auxiliary verbs in English. They list auxiliary verbs, then present the diagnostics that motivate this special class (subject-auxiliary inversion and negation with not). The modal verbs are included in this class, due to their behavior with respect to these diagnostics.
A list of verbs that (can) function as auxiliaries in English is as follows:
The status of dare, need (not), and ought (to) 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 (and not as contractions), 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 verbs (can, could, may, might, must, shall, should, will, would, and dare, need and ought when included) 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.
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.
The verbs listed in the previous section can be classified as auxiliaries based upon two diagnostics: they allow subject–auxiliary inversion (the type of inversion used to form questions etc.) and (equivalently) they can take not as a postdependent (a dependent that follows its head). The following examples illustrate the extent to which subject–auxiliary inversion can occur with an auxiliary verb but not with a full verb:
(The asterisk * is the means commonly used in linguistics to indicate that the example is grammatically unacceptable or that a particular construction has never been attested in use.) 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. Auxiliary verbs can introduce verb phrase ellipsis, but main verbs cannot. See the article on verb phrase ellipsis for examples.
Note that these criteria lead to the copula be and non-copular use of be as an existential verb being considered an auxiliary (it undergoes inversion and takes postdependent not, e.g., Is she the boss?, She is not the boss, Is there a God?, There is not a God). 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.
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 (or as full verbs). When they are light verbs, they fail the inversion and negation diagnostics for auxiliaries, e.g.
(In some cases, though, have may undergo auxiliary-type inversion and negation even when it is not used as an auxiliary verb – see Subject–auxiliary inversion § Inversion with other types of verb.)
Sometimes the distinction between auxiliary verbs and light verbs is overlooked or confused. Certain verbs (e.g., used to, have to, etc.) may be judged as light verbs by some authors, but as auxiliaries by others.
Most clauses contain at least one main verb, and they can contain zero, one, two, three, or perhaps even more auxiliary verbs. The following example contains three auxiliary verbs and one main verb:
The auxiliary verbs are in bold and the main verb is underlined. Together these verbs form a verb catena (chain of verbs), i.e., they are linked together in the hierarchy of structure and thus form a single syntactic unit. The main verb scrutinized provides the semantic core of sentence meaning, whereby each of the auxiliary verbs contributes some functional meaning. A single finite clause can contain more than three auxiliary verbs, e.g.
Viewing this sentence as consisting of a single finite clause, there are five auxiliary verbs and two main verbs present. From the point of view of predicates, each of the main verbs constitutes the core of a predicate, and the auxiliary verbs contribute functional meaning to these predicates. These verb catenae are periphrastic forms of English, English being a relatively analytic language. Other languages, such as Latin, are synthetic, which means they tend to express functional meaning with affixes, not with auxiliary verbs.
The periphrastic verb combinations in the example just given are represented now using the dependency grammar tree of the sentence; the verb catena is in green:
The particle to is included in the verb catena because its use is often required with certain infinitives. The hierarchy of functional categories is always the same. The verbs expressing modality appear immediately above the verbs expressing aspect, and the verbs expressing aspect appear immediately above the verbs expressing voice. The verb forms for each combination are as follows:
English allows clauses with both perfect and progressive aspect. When this occurs, perfect aspect is superior to progressive aspect, e.g.