In English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

, subject-auxiliary inversion (SAI) occurs when an auxiliary verb

Auxiliary verb

In linguistics, an auxiliary verb is a verb that gives further semantic or syntactic information about a main or full verb. In English, the extra meaning provided by an auxiliary verb alters the basic meaning of the main verb to make it have one or more of the following functions: passive voice,...

 precedes a subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

. This is an exception to the English word order convention of subjects preceding their corresponding verbs. See also Inversion (linguistics)

Inversion (linguistics)

In linguistics, grammatical inversion is any of a number of different distinct grammatical constructions in the languages of the world. There are three main uses in the literature which, unfortunately, have little if any overlap either formally or typologically: syntactic inversion, thematic...

.

Circumstances for inversion

Subject-auxiliary inversion occurs in the following cases:

• Questions where the answer is, generally, a "yes" or "no" response (as opposed to wh-questions
  Interrogative word
  In linguistics, an interrogative word is a function word used for the item interrupted in an information statement. Interrogative words are sometimes called wh-words because most of English interrogative words start with wh-...
  
  ); such questions are also known as polar questions. Yes-no questions must typically begin with an auxiliary verb.

Example #1: Did you finish your homework?

Example #2: Are you going to the store?

Example #3: Should I answer the phone?

• Wh-questions with auxiliary verbs.

Example #1: When did you come back?

Example #2: What did he do with the car?

Unlike yes-no questions, wh-questions in which the matrix subject is questioned can be formed without inversion of an auxiliary verb.

Example: Who ate all the pies?

• Declarative sentences with negative elements (i.e. never or not) are formed. See also Negative inversion.

Example #1: Never again will I watch that opera!

Example #2: Not since childhood did she eat cotton candy.

• Declarative sentences with restrictive elements (i.e. only or so) are formed.

Example #1: Only on Fridays does he go to the bar.

Example #2: So hard did she work that she overslept the next day.

Example #3: So did I.

SAI also occurs in tag questions (He left, didn't he?), imperatives with overt subjects (Don't anybody move!), certain counterfactuals (Had the general not ordered the advance, the front wouldn't have fallen), and, optionally, in certain comparatives (She knows more languages than does her father) and related constructions.

SAI is usually thought to be a syntactic remnant in modern English of the more widespread verb-second (V2) phenomenon found in earlier stages of the language (and in all the other Germanic languages still), and still attested in archaic prose such as:

• Example: Uneasy lies the head that wears a crown.