An auxiliary verb (abbreviated aux) is a verb that adds functional or grammatical meaning to the clause in which it appears, such as to express tense, aspect, modality, voice, emphasis, etc. Auxiliary verbs usually accompany a main verb. The main verb provides the main semantic content of the clause. An example is the verb have in the sentence I have finished my lunch. Here, the main verb is finish, and the auxiliary have helps to express the perfect aspect. Some sentences contain a chain of two or more auxiliary verbs. Auxiliary verbs are also called helping verbs, helper verbs, or (verbal) auxiliaries.
Below are some sentences that contain representative auxiliary verbs from English, Spanish, German, and French, with the auxiliary verb marked in bold:
- a. Do you want tea? – do is an auxiliary accompanying the main verb want, used here to form a question – see do-support.
- b. He has given his all. – has is an auxiliary used in expressing the perfect aspect of give.
- d. Das wurde mehrmals gesagt. – wurde 'became' is an auxiliary used to build the passive voice in German.
- That became many times said = 'That was said many times.'
- d. Das wurde mehrmals gesagt. – wurde 'became' is an auxiliary used to build the passive voice in German.
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 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.,
- The bird is in the tree. – is serves as a copula with a predicative expression not containing any other verb.
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.
List of auxiliaries in English
- be (am, are, is, was, were, being, been), can, could, dare, do (does, did), have (has, had, having), may, might, must, need, ought, shall, should, will, would
The status of dare (not), 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.
Auxiliary verb Meaning contribution Example be1 copula (= linking verb) She is the boss. be2 progressive aspect He is sleeping. be3 passive voice They were seen. can1 deontic modality I can swim. can2 epistemic modality Such things can help. could1 deontic modality I could swim. could2 epistemic modality That could help. dare deontic modality I dare not attempt it. do1 do-support/emphasis You did not understand. do2 question Do you like it? have perfect aspect They have understood. may1 deontic modality May I stay? may2 epistemic modality That may take place. might epistemic modality We might give it a try. must1 deontic modality You must not mock me. must2 epistemic modality It must have rained. need deontic modality You need not water the grass. ought deontic modality You ought to play well. shall deontic modality You shall not pass. should1 deontic modality You should listen. should2 epistemic modality That should help. will1 epistemic modality We will eat pie. will2 future tense The sun will rise tomorrow at 6:03. will3 habitual aspect He will make that mistake every time. would1 epistemic modality Nothing would accomplish that. would2 future-in-the-past tense After 1990, we would do that again. would3 habitual aspect Back then we would always go there.
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.
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 (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:
- a. He was working today.
- b. Was he working today? - Auxiliary verb was allows subject–auxiliary inversion.
- a. He worked today.
- b. *Worked he today? - Full verb worked does not allow subject–auxiliary inversion.
- a. She can see it.
- b. Can she see it? - Auxiliary verb can allows subject–auxiliary inversion.
- a. She sees it.
- b. *Sees she it? - Full verb sees does not allow subject–auxiliary inversion.
(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. Sam would try that.
- b. Sam would not try that. - The negation not appears as a postdependent of the finite auxiliary would.
- a. Sam tried that.
- b. *Sam tried not that. - The negation not cannot appear as a postdependent of the finite full verb tried.
- a. Tom could help.
- b. Tom could not help. - The negation not appears as a postdependent of the finite auxiliary could.
- a. Tom helped.
- b. *Tom helped not. - The negation not cannot appear as a postdependent of the finite full verb helped.
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.
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 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.
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. (Examples: He will come tomorrow, she must do it at once, not
he wills or she musts.)
Vs. 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 (or as full verbs). When they are light verbs, they fail the inversion and negation diagnostics for auxiliaries, e.g.
Note that in some dialects (for example, the West and South West dialects of Hiberno-English), the inversion test may sound correct to native speakers.
- a. They had a long meeting.
- b. *Had they a long meeting? - Light verb had fails the inversion test.
- c. *They had not a long meeting. - Light verb had fails the negation test.
- a. She did a report on pandering politicians.
- b. *Did she a report on pandering politicians? - Light verb did fails the inversion test.
- c. *She did not a report on pandering politicians. - Light verb did fails the negation test.
(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.
- The paper will have been scrutinized by Fred.
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.
- Fred may be being judged to have been deceived by the explanation.
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:
Functional meaning Verb combination Example Modality finite modal verb + infinitive may be Perfect aspect form of auxiliary verb have + perfect active participle have been Progressive aspect form of auxiliary verb be + progressive active participle be being Passive voice form of auxiliary verb be + passive participle been deceived
English allows clauses with both perfect and progressive aspect. When this occurs, perfect aspect is superior to progressive aspect, e.g.
- The Oxford English Dictionary, Second Edition, defines an auxiliary verb as "a verb used to form the tenses, [grammatical mood/moods], [grammatical voice/voices], etc. of other verbs." OED Second Edition, 1989. Entry for auxiliary.
- Concerning the use of coger as an auxiliary in Spanish, see for instance https://chimichurris1ba.files.wordpress.com/2013/09/manual-sintaxis-1c2ba-bachillerato.pdf.
- Concerning the use of werden as an auxiliary in German, see for instance Engel (1994:114).
- Concerning sein as an auxiliary in German used to form perfect tense/aspect, see Eroms (2000:138f.)
- Concerning the selection of avoir or être as the auxiliary verb to form perfect tense/aspect in French, see Rowlett (2007:40f.).
- Concerning être as the auxiliary used to build the passive voice in French, see Rowlett (2007:44f.).
- Concerning auxiliaries forming a closed class, see Kroeger (2004:251).
- That the equivalents of have and be are perhaps the most widely acknowledged auxiliaries across languages (related to English) can be verified by glancing at the literature on auxiliaries, e.g., Engel (1994:104ff.), Eroms (2000:137ff.), Rowlett (2007:24ff.).
- Concerning the term semi-auxiliaries for French, see Warnant (1982:279).
- For lists of the auxiliary verbs like the one produced here but with minor discrepancies, see for instance Radford (2004:324), Crystal (1997:35), and Jurafsky and Martin (2000:322).
- For some discussion of the status of dare as a "marginal modal", see Fowler's Modern English Usage, p. 195f.
- For examples of the inversion diagnostic used to identify auxiliaries, see for instance Radford (1997:50f., 494), Sag and Wasow (1999:308f.), and Kroeger (2004:253).
- The negation diagnostic for identifying auxiliary verbs is employed for instance by Radford (1997:51), Adgar (2003:176f.), and Culicover (2009:177f.).
- Jurafsky and Martin (2000:320) state clearly that copula be is an auxiliary verb. Bresnan (2001:18f.) produces and discusses examples of subject-auxiliary inversion using the copula. Tesnière (1959) repeatedly refers to the copula être in French as an auxiliary verb, and Eroms (2000:138f.) discusses the copula sein in German as a Hilfsverb 'helping verb'. Crystal (1997:35) lists be as an auxiliary verb without distinguishing between its various uses (e.g., as a copula or not). Other definitions are less clear; Radford (2004:324) suggests that copula be is not an auxiliary, but he does not address why it behaves like an auxiliary with respect to the criteria he employs (e.g., inversion) for identifying auxiliaries.
- Concerning light verbs in English, see Allterton (2006:176).
- Light verbs are called Funktionsverben 'function verbs' in German - see Engel (1994:105f.) and Eroms (2000:162ff.).
- Jurafsky and Martin (2000:22), for instance, lists have as a modal auxiliary when it appears as have to and Fowler's Modern English Usage (1996:195) lists used to as a "marginal modal".
- See Finch (2000:13) concerning the necessity that a given auxiliary verb should accompany a main verb.
- Dependency trees like the ones here can be found, for instance, in Osborne and Groß (2012) Take note that other authors consider closed-class words to be the heads of open-class complements, for instance determiners heads of common nouns. These dependency trees represent the opinion of the authors and not of all Dependency Grammar authorities.
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