Do-support
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 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 (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". Common uses. "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. 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, "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 (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. 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 "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. 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" (negative inversion): 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 (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: 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": 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). 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 (see below section #Pro-verbs & Do-so Substitution 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!" 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: 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: With a simple sentence: Here again exemplified by Santorini and Kroch, "do so" substitution for testing constituent verb phrases in the above sample sentence: 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 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: 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 (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. 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 (in English, -ed) 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.