Subject–auxiliary inversion

Subject–auxiliary inversion (SAI; also called subject–operator inversion) is a frequently occurring type of inversion in the English language 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 the subject–verb inversion in English.