In English grammar do-support or do-insertion refers to the use of the auxiliary verb do in negative or interrogative clauses that do not contain other auxiliaries.
While the English word do may also serve as a main verb, as in "We do the laundry on Sundays", do support occurs only in the presence of another verb serving as the main verb, as for example in the conversion of I go there to either the question form Do I go there? or the negative form I do not go there.
Except in copular sentences such as Is he here? or They are not banjo players (and except with the main verb "have", as in Have you any bananas? or I haven't any bananas, which are permissible in some but not all dialects), in English almost all questions and almost all sentences with negative polarity feature an auxiliary verb. Questions and negative sentences that do not include a modal auxiliary or a form of have or be include a form of do. This auxiliary do is inflected for person, number, and tense, and may be contracted with not so the latter becomes n't, as in don't, doesn't and didn't. In questions and negatives the auxiliary do has no meaning in itself,[1] so it is sometimes called a dummy auxiliary.
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Negative sentences in English are formed by placing the negative adverb not after the first auxiliary verb in the sentence.[2] If a sentence has no other auxiliary, a form of the word do is placed where the first auxiliary would otherwise be, with not or n't following it.[2] Compare the following two sentences:
In the first example, the verb flirt appears in the past tense. In the second, related sentence, the past tense form of do, did, and the word not appear before the plain form of the verb. One may think of the past tense as having moved to a position before not, where the auxiliary do is inserted in order to carry it.[2]
Simple yes-no questions in English are usually formed by placing the first auxiliary at the front of the sentence, before the subject. If there is no other auxiliary, do occurs in this position.[2]
Occasionally yes-no questions are formed without grammatical changes and hence without do support, by the use of intonation or punctuation alone, as in "John rides the bus?"
So-called Wh-questions, containing an interrogative word such as who, what, when, where, why, how, or the like, show the same pattern of subject-auxiliary inversion seen in yes-no questions, and use similar do-support.
Tag questions are formed with a copy of the first auxiliary and a pronoun version of the sentence's subject at the end of the sentence.[2] As with other questions, sentences that contain no other auxiliary use do-support.