In the grammar of English, the term do-support (or do-insertion) refers to the use of the auxiliary verb do, including its inflected forms does and did, to produce 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, where 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 allowable (in Modern English) to add the negating word not to an ordinary finite lexical verb, as in *I know not – it can only be added to an auxiliary (or copular) verb, 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 possible to say *Know you him?; grammatical rules require Do you know him?
Do-support is not used when there is already an auxiliary or copular verb present; nor is it used with non-finite verb forms (infinitives and participles) or 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.
Common uses 
Do-support appears to accommodate a number of varying grammatical constructions, e.g. 1) question formation, 2) the appearance of the negation not, and 3) negative inversion. These constructions often cannot occur without do-support, or the presence of some other auxiliary verb.
In questions 
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, then do-support is not required when forming questions:
- He will laugh. → Will he laugh? (the auxiliary will inverts with the subject he)
- She is at home. → Is she at home? (the copula is inverts with the subject she)
- When will he laugh?
However, if there is no auxiliary or copula present, inversion requires the introduction of an auxiliary in the form of do-support:
- I know. → Do I know? (Compare: *Know I?)
- He laughs. → Does he laugh? (Compare: *Laughs he?)
- She came home. → When did she come home? (Compare: *Came she home?)
Note that the finite (inflected) verb is now the auxiliary do; the following verb is a bare infinitive which does not inflect. Hence 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. This applies both to do-support and to other auxiliaries:
- Why are you not playing? / Why aren't you playing?
- Do you not want to try? / Don't you want to try?
The above principles do not apply to wh-questions where the interrogative word is the subject or part of the subject. In these cases, there is no inversion and hence no need for do-support: Who lives here?, Whose dog bit you?
For elliptical questions and tag questions, see the elliptical sentences section below.
With the negation not 
In the same way that the presence of an auxiliary allows question formation, and it also allows the appearance of the negating word not. In this case too, if no other auxiliary or copular verb is present, do-support is required.
- He will laugh. → He will not laugh. (not attaches to the auxiliary will)
- She laughs. → She does not laugh. (not attaches to the added auxiliary does)
In the second sentence, do-support is required because modern English does not allow forms like *She laughs not (although such constructions are frequent in archaic English).
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. These forms are used very frequently in informal English, e.g.
Do-support is required for negated imperatives, even when the verb is the copula be:
- Do not do that.
- Don't be silly.
However, there is no do-support with non-finite and subjunctive verb forms, as these are negated by a preceding not:
- It would be a crime not to help him (the infinitive to help is negated)
- Not knowing what else to do, I stood my ground (the present participle knowing is negated)
- Not eating vegetables can harm your health (the gerund eating is negated)
- I suggest that he not receive any more funding (the subjunctive receive is negated)
The negation in these examples is negating the non-finite predicate. Compare the following competing formulations:
- I did not try to laugh. vs. I tried not to laugh.
- They do not want to go. vs. They want to not go.
There are two predicates in each of the verb chains in these sentences, e.g. try and laugh in the first example. Do-support is needed when the higher of the two is negated; it is not needed to negate the lower nonfinite predicate.
In cases of 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:
- Never did he run that fast again. (Compare: *Never he did run that fast again. *Never ran he that fast again.)
- Only here do I feel at home. (Compare: ?Only here I feel at home.)
Further uses of auxiliary do 
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 strictly speaking do not require do-support. In such cases, do-support optionally appears for pragmatic reasons. The following subsections describe 1) the use of do-support to express emphasis, and 2) the appearance of do-support to enable VP-ellipsis.
For emphasis 
In this case the auxiliary generally appears for purposes of emphasis, for instance to establish a contrast or to express a correction. For example:
- Did Bill eat his breakfast? Yes, he did eat his breakfast. (auxiliary did emphasizes the positive answer, which may be unexpected)
- Bill doesn't sing, then. No, he does sing. (auxiliary does emphasizes the correction of the previous statement)
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, this usage of do does not occur with other auxiliaries or a copular verb. In these cases, emphasis can be obtained by adding stress to the auxiliary or copular:
- Would you take the risk? Yes, I would take the risk.
- Bill isn't singing, then. No, he is singing.
(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. This applies whether or not do-support is used:
- I wouldn't (or would not) take the risk.
- They don't (or do not) appear on the list.
Emphatic do can also be used with imperatives, including with the copula be:
- Do take care! Do be careful!
In elliptical sentences 
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). This includes the following types:
- Tag questions:
- He plays well, doesn't he?
- You don't like Sara, do you?
- Elliptical questions:
- I like pasta. Do you?
- I went to the party. Why didn't you?
- Elliptical statements:
- They swam, but I didn't.
- He looks smart, and so do you.
- You fell asleep and I did too.
Such uses include cases where do-support would have been used in a complete clause (questions, negatives, inversion), but also cases where (as in the last example) the complete clause would normally have been constructed without do (in this case, 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: in this case did substitutes for fell asleep.
As in the principal cases of do-support, this usage of do does not normally occur when there is already an auxiliary or copula present; here the auxiliary or copula is retained in the elliptical sentence. For example
- He is playing well, isn't he?
- I can cook pasta. Can you?
- You should get some sleep, and I should too.
However it is possible to use do as a pro-verb even after auxiliaries:
- Have you put the shelf up yet? I haven't done (or I haven't), but I will do (or I will).
(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!
Use of do as main verb 
Apart from its uses as an auxiliary, the verb do (with its inflected forms does, did, done, doing) can also be used as an ordinary lexical verb (main verb):
- Do your homework!
- What are you doing?
In this use, like other non-auxiliary verbs, do cannot be directly negated with not and cannot participate in inversion. Hence it may itself require do-support, with both auxiliary and lexical instances of do appearing together:
- They didn't do the laundry on Sunday. (did is the auxiliary, do is the main verb)
- Why do you do karate? (the first do is the auxiliary, the second is the main verb)
Meaning contribution 
When the auxiliary verb do appears as just illustrated, it does not contribute in any obvious way to the meaning of the sentence,  so it is sometimes called a dummy auxiliary. Whether this is the correct way to understand the role of auxiliary do in such cases is debatable, however. From a diachronic point of view, the auxiliary do may have been an aspect marker, which means it did originally convey functional meaning, i.e. aspectual meaning.
See also 
- Kaplan, Jeffrey P. (1989), English Grammar: Principles and Facts, Englewood Cliffs, New Jersey: Prentice-Hall
- Huddleston, Rodney D.; Geoffrey K. Pullum (2005). A Student's Introduction To English Grammar. Cambridge U Press.
- DeCapua, Andrea (2008). Grammar for Teachers. Springer.
- Heidinger, Virginia (1984). Analyzing Syntax and Semantics. Gallaudet U Press.
- Traugott, Elizabeth Closs; Mary Louise Pratt (1980), Linguistics for Students of Literature, San Diego: Harcourt Brace Jovanovich