Do-support

[]Do-support in English grammar is the use of the auxiliary verb do to form negated clauses and constructions which require subject–auxiliary inversion, such as [|questions].
The verb do can be used optionally as an auxiliary even in simple declarative sentences, usually as a means of adding emphasis. However, in negated and inverted clauses, do is usually used in today's Modern English. For example, in idiomatic English, the negating word not cannot attach directly to just any finite lexical verb; rather, it can only attach 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 the finite lexical verb know, while grammatical, is archaic. If there is no other auxiliary present when negation is required, the auxiliary do is used to produce a form like I do not 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. 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.

Common uses

Do-support appears to accommodate a number of varying grammatical constructions:

question formation,
the appearance of the negation not, and
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 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:
This applies not only in yes–no questions but also in questions formed using interrogative words:
However, if there is no auxiliary or copula present, inversion requires the introduction of an auxiliary in the form of do-support:
The finite verb is now the auxiliary do; the following verb is a bare infinitive which does not inflect: does he laugh? ; did she come?.
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 to both do-support and 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.

With ''not''

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 idiomatic Modern 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 suffix -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 verb forms, as they are negated by a preceding not:
With subjunctive verb forms, such 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 it is be :
As a past subjunctive, however, did is needed for negation :
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.
For negated questions, see the questions section above. For negated elliptical sentences, see the elliptical sentences section below.

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 :

Further uses

In addition to providing do-support in questions and negative clauses, the auxiliary verb do can also appear in contexts where it is not grammatically required. In such cases, its use often serves pragmatic functions, such as emphasis or contrast.

For emphasis

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.
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:
In negative sentences, emphasis can be obtained by adding stress either to the negating word 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:

In elliptical sentences

The auxiliary do is also used in various types of elliptical sentences, where the main verb is omitted. That 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 answers:
* Do you want to come along? — I do.
* Who took the car? — He did.
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 that do-support would have been used in a complete clause but also cases that the complete clause would normally have been constructed without do. 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 , but I will do.
Pro-verbal uses of do are also found in the imperative:

''Please do. — Don't!''
Pro-verbs and do-so substitution

The phrases do so and do what for questions are pro-verb forms in English. They can be used as substitutes for verbs in x-bar theory grammar to test verb phrase completeness. Bare infinitives forms often are used in place of the missing pro-verb forms.
Examples from Santorini and Kroch:

Type	Sample	Sample w/ Replacement
Substitution	She will write a book.	✓ She will 'do so'.
Substitution	The two boys could 'order tuna salad sandwiches'.	✓ The two boys could 'do so'.
Question/short answer	'What' will she 'do'?	✓ 'Write a book'.
Question/short answer	'What' could the two boys 'do'?	✓ 'Order tuna salad sandwiches'.

Tests for constituenthood of a verb-phrase in X'-grammar

The do so construction can be used to test if a verb-phrase is a constituent phrase in X'-grammar by substitution similarly to how other pro-forms can be used to test for noun-phrases, etc.
In X-bar theory, the verb-phrase projects three bar-levels such as this:


 VP
 / \
 ZP X'
 / \
 X' YP
 |
 X 
 |
 head

With a simple sentence:


 S
 |
 VP
 / \
 / \
 / \
 / \
 NP \
 / \ \ 
 DP N' V'
 | | / \
The children / \
 / \
 V' PP
 / \ /_\
 / \ with gusto
 V NP
 | /_\
 ate the pizza

Here again exemplified by Santorini and Kroch, do so substitution for testing constituent verb phrases in the above sample sentence:


 S
 |
 VP
 / \
 / \
 / \
 / \
 NP \
 / \ \ 
 DP N' V'
 | | / \
The children / \
 / \
 V' PP
 / \ /_\
 / \ with gusto
 V NP
 | /_\
 did so the pizza

Use of ''do'' as main verb

Apart from its uses as an auxiliary, the verb do can be used as an ordinary lexical 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: