Do-support


[]Do-support, 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 conventions of Modern 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. 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:
  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 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. 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.

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 -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 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 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.

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:
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 & 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:
TypeSampleSample w/ Replacement
SubstitutionShe will write a book.✓ She will 'do so'.
SubstitutionThe 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:

Meaning contribution

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. In Early Modern English, the semantic value was lost, and the usage of forms with do began to approximate that found today.

Origins

Some form of auxiliary "do" occurs in all West Germanic languages except Afrikaans. It is generally accepted that the past tense of Germanic weak verbs was formed from a combination of the infinitive with a past tense form of "do", as exemplified in Gothic. The origins of the construction in English are debated: some scholars argue it was already present in Old English, but not written due to stigmatization. Scholars disagree whether the construction arose from the use of "do" as a lexical verb in its own right, or whether periphrastic "do" arose from a causative meaning of the verb or vice versa. Examples of auxiliary "do" in Old English writing appear to be limited to its use in a causative sense, which is parallel to the earliest uses in other West Germanic languages. Others argue that the construction arose either via the influence of Celtic speakers or that the construction arose as a form of creolization when native speakers addressed foreigners and children.