Do-support

Do-support (or do-insertion), in English grammar, 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, 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 rules of 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. 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 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.

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

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