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Do-support (sometimes referred to as do-insertion or periphrastic do), in English grammar, is the use of the auxiliary verb do (or one of its inflected forms e.g. does), 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 (e.g. "I did shut the fridge."). 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 (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.