Subject-auxiliary inversion is a common occurring type of inversion in English.
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.
Examples of subject–auxiliary inversion:
Mary has watched the movie.
Has Mary watched the movie? (Yes-no question formed using inversion)
Here the subject is Mary, and the verb has is an auxiliary. In the question, these two elements change places. 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:
Mary enjoys the movie. (Statement with the non-auxiliary verb enjoys)
*Enjoys Mary the movie? (This is incorrect; simple inversion not possible with this type of verb)
Does Mary enjoy the movie? (The sentence formulated with the auxiliary does now allows inversion)
It is also possible for the subject to invert with a negative contraction.
He isn’t nice.
Isn’t he nice?
How to Use Subject-Auxiliary Inversion
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:
Mary has watched the movie.
Has Mary watched the movie?
And also in questions introduced by other interrogative words:
Mary is watching the movie.
What is Mary watching?
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:
Somebody has watched the movie.
Who has watched the movie?
Which fool has watched the movie?
Inversion also does not normally occur in indirect questions.
“What did Mary watch?“, Alice wonders. (Inversion in a direct question)
*Linda wonders what did Mary watch. (Incorrect; inversion should not be used in an indirect question)
Linda wonders what Mary watched. (Correct; indirect question formed without inversion)
We asked whether James had arrived. (Correct; indirect question without inversion)
*We asked whether had James arrived. (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.)
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.
He went abroad last year, and I did too.
He went abroad last year, and so did I.
The same frequently occurs in elliptical clauses beginning with as.
He went abroad last year, as did I.
Inversion also occurs following an expression beginning with so or such, as in:
I felt so sad that I could cry.
So sad did I feel that I could cry.
Subject-auxiliary inversion may optionally be used in elliptical clauses introduced by the particle of comparison than:
Cindy has more skirts than her sister does.
Cindy has more skirts than does her sister.