Do-supportwikipedia
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.
do''-supportdo-supportinsertedauxiliary ''dothe auxiliary is taken from the emphatic ''do'' formdosupportstandard English has itDo-periphrasis

Auxiliary verb

auxiliary verbauxiliaryauxiliary verbs
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.
– do is an auxiliary accompanying the main verb want, used here to form a question – see do-support.

Subject–auxiliary inversion

subject–auxiliary inversionsubject-auxiliary inversioninversion
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 presence of an auxiliary (or copular) verb allows subject–auxiliary inversion to take place, as is required in most interrogative sentences in English. 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):
For details of the use of do, did and does for this and similar purposes, see do-support.

Question

questionwh-questionanswer
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.
However, English allows such inversion only with a particular class of verbs (called auxiliary or special verbs), and thus sometimes requires the addition of an auxiliary do, does or did before inversion can take place ("He sings" → "Does he sing?") – for details see do-support.

Imperative mood

imperativeimperative moodimperatives
Do-support is required for negated imperatives even when the verb is the copula be:
(The present subjunctive always has the same form as the imperative, although it is negated differently – the imperative is negated using do not, as in "Don't touch me!"; see do-support.) The imperative form is understood as being in the second person (the subject pronoun you is usually omitted, although it can be included for emphasis), with no explicit indication of singular or plural.

Syntax

syntaxsyntacticsyntactical
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.

Modern English

modern EnglishEnglishModern
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.

English grammar

Englishgrammarthere is
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.
For more details of this, see do-support.

Infinitive

infinitiveto''-infinitivebare infinitive
Do-support is not used when there is already an auxiliary or copular verb present or with non-finite verb forms (infinitives and participles).
(Periphrases can be employed instead in some cases, like (to) be able to for can, and (to) have to for must.) It also applies to the auxiliary do, like used in questions, negatives and emphasis like described under do-support.

English clause syntax

frontingconditional
However such inversion is only possible with an auxiliary or copular verb; if no such verb would otherwise be present, do-support is used.

Interrogative

interrogativeinterrogative sentenceinterrogative mood
The presence of an auxiliary (or copular) verb allows subject–auxiliary inversion to take place, as is required in most interrogative sentences in English.
In sentences where no such verb is otherwise present, the auxiliary do (does, did) is introduced to enable the inversion (for details see do-support, and.

Tag question

tag questiontag questionsquestion tag
If the verb is in the present perfect, for example, the tag question uses has or have; if the verb is in a present progressive form, the tag is formed with am, are, is; if the verb is in a tense which does not normally use an auxiliary, like the present simple, the auxiliary is taken from the emphatic do form; and if the sentence has a modal auxiliary, this is echoed in the tag:

Negative inversion

negative inversion
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):

English verbs

-edEnglishEnglish regular verbs
Other verbs used as auxiliaries include have, chiefly in perfect constructions (the forms has, have and had can contract to 's, 've and 'd), and do (does, did) in emphatic, inverted and negated constructions (see do-support).

Do

dodare

English auxiliaries and contractions

auxiliary verbcontractedauxiliaries
Also, "Do" does help in the formation of questions, negations, etc., as described in the article on do-support.

English modal verbs

modal verbsshalldouble modal
More common, however, (though not the most formal style) is the syntax that treats used as a past tense of an ordinary verb, and forms questions and negatives using did: ''Did he use(d) to come here?

Inverted sentence

inverted sentence
In English, such an inversion often introduces do-support.

English language

EnglishEnglish-languageen
Some changes, such as the use of do-support have become universalised.

Uses of English verb forms

past progressivesimplefuture-in-the-past
Negation uses do-support (i.e. do not or don't).

Affirmation and negation

negationnegativepolarity
If no such verb is present then the dummy auxiliary do (does, did) is introduced – see do-support.