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:
For details of the use of do, did and does for this and similar purposes, see do-support. For exceptions to the principle that the inverted verb must be an auxiliary, see § Inversion involving non-auxiliary verbs below. It is also possible for the subject to invert with a negative contraction (can't, isn't, etc.). For example:
Compare this with the uncontracted form Is he not nice? and the archaic Is not he nice?.
The main uses of subject-auxiliary inversion in English are described in the following sections, although other types can occasionally be found. 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.
The most common use of subject-auxiliary inversion in English is in question formation. It appears in yes-no questions:
and also in questions introduced by other interrogative words (wh-questions):
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):
Another use of subject-auxiliary inversion is in sentences which begin with certain types of expressions which contain a negation or have negative force. For example,
This is described in detail at negative inversion.
Subject-auxiliary inversion can be used in certain types of subordinate clause expressing a condition:
Note that when the condition is expressed using inversion, the conjunction if is omitted. More possibilities are given at English conditional sentences § Inversion in condition clauses, and variations are described at English subjunctive § Inversion.
Inversion also occurs following an expression beginning with so or such, as in:
Subject-auxiliary inversion may optionally be used in elliptical clauses introduced by the particle of comparison than:
There are certain sentence patterns in English in which subject-verb inversion takes place where the verb is not restricted to an auxiliary verb. Here the subject may invert with certain main verbs, e.g. After the pleasure comes the pain, or with a chain of verbs, e.g. In the box will be a bottle. These are described in the article on subject-verb inversion. Further, inversion was not limited to auxiliaries in older forms of English. Examples of non-auxiliary verbs being used in typical subject-auxiliary inversion patterns may be found in older texts or in English written in an archaic style:
The verb have, when used to denote broadly defined possession (and hence not as an auxiliary), is still sometimes used in this way in modern standard English:
In some cases of subject-auxiliary inversion, such as negative inversion, the effect is to put the finite auxiliary verb into second position in the sentence. In these cases, inversion in English results in word order that is like the V2 word order of other Germanic languages (Danish, Dutch, Frisian, Icelandic, German, Norwegian, Swedish, Yiddish, etc.). These instances of inversion are remnants of the V2 pattern that formerly existed in English as it still does in its related languages. Old English followed a consistent V2 word order.
The structural analysis of subject-auxiliary inversion, and of inversion in general, challenges many theories of sentence structure, in particular, those theories based on phrase structure. The challenge stems from the fact that these theories posit the existence of a finite verb phrase constituent. The standard declarative sentence is divided into two immediate constituents, a subject NP and a predicate VP. When subject-auxiliary inversion occurs, it appears to violate the integrity of the predicate. The canonical predicate is underlined in the following sentences:
The finite VP predicate is a continuous sequence of words in the a-sentences. In the b-sentences in contrast, subject-auxiliary inversion breaks up the predicate. What this means is that in one sense or another, a discontinuity is present in the structure.
One widespread means of addressing this difficulty is to posit movement. The underlying word order of the b-sentences is deemed to be that shown in the a-sentences. To arrive at the inversion word order in the b-sentences, movement is assumed. The finite verb moves out of its base position after the subject into a derived position in front of the subject.
By moving out of its base position and into the derived position at the front of the clause, the integrity of the predicate VP constituent can be maintained, since it is present at an underlying level of sentence structure.
An alternative analysis does not acknowledge the binary division of the clause into subject NP and predicate VP, but rather it places the finite verb as the root of the entire sentence and views the subject as switching to the other side of the finite verb. No discontinuity is perceived. Dependency grammars are likely to pursue this sort of analysis. The following dependency trees illustrate how this alternative account can be understood:
These trees show the finite verb as the root of all sentence structure. The hierarchy of words remains the same across the a- and b-trees. If movement occurs at all, it occurs rightward (not leftward); the subject moves rightward to appear as a post-dependent of its head, which is the finite auxiliary verb.