Do-support (or do-insertion), in English grammar, is the use of the auxiliary verb do, including its inflected forms does and did, to form negated clauses and questions as well as other constructions in which subject-auxiliary inversion is required.
The verb "do" can be used as an auxiliary even in simple declarative sentences, and it usually serves to add emphasis, as in "I did shut the fridge." However, in the negated and inverted clauses referred to above, it is used because the rules of English syntax permit these constructions only when an auxiliary is present. It is not idiomatic in Modern English to add the negating word not to a lexical verb with finite form; not can be added only to an auxiliary or copular verb. For example, the sentence I am not with the copula be is fully idiomatic, but I know not with a finite lexical verb, while grammatical, is archaic. Hence if there is no other auxiliary present when negation is required, the auxiliary do is used to produce a form like I do not (don't) know. The same applies in clauses requiring inversion, including most questions: inversion must involve the subject and an auxiliary verb so it is not idiomatic to say Know you him?; today's English usually substitutes Do you know him?
Do-support is not used when there is already an auxiliary or copular verb present or with non-finite verb forms (infinitives and participles). It is sometimes used with subjunctive forms. Furthermore, the use of do as an auxiliary should be distinguished from the use of do as a normal lexical verb, as in They do their homework.
Do-support appears to accommodate a number of varying grammatical constructions:
These constructions often cannot occur without do-support or the presence of some other auxiliary verb.
The presence of an auxiliary (or copular) verb allows subject-auxiliary inversion to take place, as is required in most interrogative sentences in English. If there is already an auxiliary or copula present, do-support is not required when forming questions:
However, if there is no auxiliary or copula present, inversion requires the introduction of an auxiliary in the form of do-support:
The finite (inflected) verb is now the auxiliary do; the following verb is a bare infinitive which does not inflect: does he laugh? (not laughs); did she come? (not came).
In negated questions, the negating word not may appear either following the subject, or attached to the auxiliary in the contracted form n't. That applies both to do-support and to other auxiliaries:
The above principles do not apply to wh-questions if the interrogative word is the subject or part of the subject. Then, there is no inversion and so there is no need for do-support: Who lives here?, Whose dog bit you?
The verb have, in the sense of possession, is sometimes used without do-support as if it were an auxiliary, but this is considered dated. The version with do-support is also correct:
For elliptical questions and tag questions, see the elliptical sentences section below.
In the same way that the presence of an auxiliary allows question formation, the appearance of the negating word not is allowed as well. Then too, if no other auxiliary or copular verb is present, do-support is required.
In the second sentence, do-support is required because Modern Idiomatic English does not allow forms like *She laughs not. The verb have, in the sense of possession, is sometimes negated thus:
Most combinations of auxiliary/copula plus not have a contracted form ending in -n't, such as isn't, won't, etc. The relevant contractions for negations formed using do-support are don't, doesn't and didn't. Such forms are used very frequently in informal English.
Do-support is required for negated imperatives even when the verb is the copula be:
However, there is no do-support with non-finite, as they are negated by a preceding not:
With subjunctive verb forms, as a present subjunctive, do is infrequently used for negation, which is frequently considered ambiguous or incorrect because it resembles the indicative. The usual method to negate the present subjunctive is to precede the verb with a not, especially if the verb is be (as do-support with it, whether it be indicative or subjunctive, is ungrammatical):
As a past subjunctive, however, did is needed for negation (unless the verb is be, whose past subjunctive is were):
The negation in the examples negates the non-finite predicate. Compare the following competing formulations:
There are two predicates in each of the verb chains in the sentences. Do-support is needed when the higher of the two is negated; it is not needed to negate the lower nonfinite predicate.
The same principles as for question formation apply to other clauses in which subject-auxiliary inversion is required, particularly after negative expressions and expressions involving only (negative inversion):
In addition to providing do-support in questions and negated clauses as described above, the auxiliary verb do can also be used in clauses that do not require do-support. In such cases, do-support may appear for pragmatic reasons.
The auxiliary generally appears for purposes of emphasis, for instance to establish a contrast or to express a correction:
As before, the main verb following the auxiliary becomes a bare infinitive, which is not inflected (one cannot say *did ate or *does sings in the above examples).
As with typical do-support, that usage of do does not occur with other auxiliaries or a copular verb. Then, emphasis can be obtained by adding stress to the auxiliary or copular:
(Some auxiliaries, such as can, change their pronunciation when stressed; see Weak and strong forms in English.)
In negative sentences, emphasis can be obtained by adding stress either to the negating word (if used in full) or to the contracted form ending in n't. That applies whether or not do-support is used:
Emphatic do can also be used with imperatives, including with the copula be:
The auxiliary do is also used in various types of elliptical sentences, where the main verb is omitted (it can be said to be "understood", usually because it would be the same verb as was used in a preceding sentence or clause). That includes the following types:
Such uses include cases that do-support would have been used in a complete clause (questions, negatives, inversion) but also cases that (as in the last example) the complete clause would normally have been constructed without do (I fell asleep too). In such instances do may be said to be acting as a pro-verb since it effectively takes the place of a verb or verb phrase: did substitutes for fell asleep.
As in the principal cases of do-support, do does not normally occur when there is already an auxiliary or copula present; the auxiliary or copula is retained in the elliptical sentence:
However, it is possible to use do as a pro-verb even after auxiliaries in some dialects:
(However it is not normally used in this way as a to-infinitive: Have you put the shelf up? I plan to, rather than *I plan to do; or as a passive participle: Was it built? Yes, it was, not *Yes, it was done.)
Pro-verbal uses of do are also found in the imperative: Please do. Don't!
Apart from its uses as an auxiliary, the verb do (with its inflected forms does, did, done, doing) can be used as an ordinary lexical verb (main verb):
Like other non-auxiliary verbs, do cannot be directly negated with not and cannot participate in inversion so it may itself require do-support, with both auxiliary and lexical instances of do appearing together:
In the various cases seen above that require do-support, the auxiliary verb do makes no apparent contribution to the meaning of the sentence so it is sometimes called a dummy auxiliary. Historically, however, in Middle English, auxiliary do apparently had a meaning contribution, serving as a marker of aspect (probably perfective aspect, but in some cases, the meaning may have been imperfective). In Early Modern English, the semantic value was lost, and the usage of forms with do began to approximate that found today.
This feature is rare or non-existent in other Germanic languages but common in Celtic ones like Welsh and Cornish. "Do" is also more common in Celtic Englishes than Standard English. For this reason there is a hypothesis that English acquired do-support due to the influence of Celtic speakers on the spoken language.