Subject-auxiliary inversion (also called subject-operator inversion) is a frequently occurring type of inversion 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.
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Definition and Examples of Subject-Auxiliary 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 auxiliary verbs which may participate in such inversion (e.g. is, can, have, will, etc.) are described at English auxiliaries. 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.
A typical example of subject–auxiliary inversion is given below.
Jenny has read the paper. (Statement)
Has Jenny read the paper? (Yes-no question formed using inversion)
Here the subject is Jenny, 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:
Jenny enjoys the paper. (Statement with the non-auxiliary verb enjoys)
*Enjoys Jenny the paper? (This is incorrect; simple inversion not possible with this type of verb)
Does Jenny enjoy the paper? (The sentence formulated with the auxiliary does now allows inversion)
It is also possible for the subject to invert with a negative contraction (can’t, isn’t, etc.).
He isn’t nice.
Isn’t he nice? (The subject he inverts with the negated auxiliary contraction isn’t)
(Compare this with the uncontracted form Is he not nice? and the archaic Is not he nice?)
How to Use Subject-Auxiliary Inversion
The main uses of subject-auxiliary inversion in English are described in the following sections, although other types can occasionally be found. It should be noted that most of these uses of inversion are restricted to main clauses; they are not found in subordinate clauses. However other types (such as inversion in condition clauses) are specific to subordinate clauses.
Subject-Auxiliary Inversion In Questions
The most common use of subject-auxiliary inversion in English is in question formation. It appears in yes-no questions:
Jenny has read the paper. (Statement)
Has Jenny read the paper? (Question)
And also in questions introduced by other interrogative words (wh-questions):
Jenny is reading the paper. (Statement)
What is Jenny reading? (Question introduced by interrogative what)
Inversion does not occur, however, when the interrogative word is the subject or is contained in the subject. In this case the subject remains before the verb (it can be said that wh-fronting takes precedence over subject-auxiliary inversion):
Somebody has read the paper. (Statement)
Who has read the paper? (The subject is the interrogative who; no inversion)
Which fool has read the paper? (The subject contains the interrogative which; no inversion)
Inversion also does not normally occur in indirect questions.
“What did Jenny eat?“, Alice wonders. (Inversion in a direct question)
*Alice wonders what did Jenny eat. (Incorrect; inversion should not be used in an indirect question)
Alice wonders what Jenny ate. (Correct; indirect question formed without inversion)
We asked whether David had left. (Correct; indirect question without inversion)
*We asked whether had David left. (Incorrect)
Another use of subject–auxiliary inversion is in sentences which begin with certain types of expressions which contain a negation or have negative force.
Rosie will say that at no time.
At no time will Rosie say that. (Subject-auxiliary inversion with a fronted negative expression.)
This is described in detail at negative inversion.
Inversion in Condition Clauses
Subject-auxiliary inversion can be used in certain types of subordinate clause expressing a condition:
If he had trained hard, he would have won the match.
Had he trained hard, he would have won the match. (Subject-auxiliary inversion of a counterfactual conditional clause)
Note that when the condition is expressed using inversion, the conjunction if is omitted.
Subject-auxiliary inversion is used after the anaphoric particle so, mainly in elliptical sentences. The same frequently occurs in elliptical clauses beginning with as.
Max fell asleep, and Fred fell asleep too.
Fred fell asleep, and so did Max.
Fred fell asleep, as did Max.
Inversion also occurs following an expression beginning with so or such, as in:
We felt so tired (such tiredness) that we fell asleep.
So tired (Such tiredness) did we feel that we fell asleep.
Subject-auxiliary inversion may optionally be used in elliptical clauses introduced by the particle of comparison than:
Sally knows more languages than her father does.
Sally knows more languages than does her father. (Optional inversion, with no change in meaning)