Subject-auxiliary inversion

Subject–auxiliary inversion (also called subject–operator inversion) is a frequently occurring type of inversion in English, whereby a finite auxiliary verb – taken here to include finite forms of the copula be – appears to "invert" (change places) with the subject. The word order is therefore Aux-S (auxiliary–subject), which is the opposite of the canonical SV (subject–verb) order of declarative clauses in English. The most frequent use of subject–auxiliary inversion in English is in the formation of questions, although it also has other uses, including the formation of condition clauses, and in the syntax of sentences beginning with negative expressions (negative inversion).

In certain types of English sentences, inversion is also possible with verbs other than auxiliaries; these are described in the article on subject-verb inversion.

Subject–auxiliary inversion involves placing the subject after a finite auxiliary verb, rather than before it as is the case in typical declarative sentences (the canonical word order of English being subject–verb–object). The auxiliary verbs which may participate in such inversion (e.g. is, can, have, will, etc.) are described at English auxiliaries and contractions. Note that forms of the verb be are included regardless of whether or not they function as auxiliaries in the sense of governing another verb form. (For exceptions to this restriction, see § Inversion with other types of verb below.)

A typical example of subject–auxiliary inversion is given below.

Here the subject is Sam, and the verb has is an auxiliary. In the question, these two elements change places (invert). If the sentence does not have an auxiliary verb, this type of simple inversion is not possible. Instead, an auxiliary must be introduced into the sentence in order to allow inversion:

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