In linguistics, inversion is any of several grammatical constructions where two expressions switch their canonical order of appearance, that is, they invert. The most frequent type of inversion in English is subject–auxiliary inversion, where an auxiliary verb changes places with its subject; this often occurs in questions, such as Are you coming?, where the subject you is switched with the auxiliary are. In many other languages – especially those with freer word order than English – inversion can take place with a variety of verbs (not just auxiliaries) and with other syntactic categories as well.
When a layered constituency-based analysis of sentence structure is used, inversion often results in the discontinuity of a constituent, although this would not be the case with a flatter dependency-based analysis. In this regard inversion has consequences similar to those of shifting.
Inversion in English 
In broad terms, one can distinguish between two major types of inversion in English that involve verbs: subject–auxiliary inversion and subject–verb inversion. The difference between these two types resides with the nature of the verb involved, i.e. whether it is an auxiliary verb or a full verb.
Subject–auxiliary inversion 
- a. Fred will stay.
- b. Will Fred stay? - Subject–auxiliary inversion with yes/no question
- a. Larry has done it.
- b. What has Larry done? - Subject–auxiliary inversion with constituent question
- a. Fred has helped at no point.
- b. At no point has Fred helped. - Subject–auxiliary inversion with fronted expression containing negation (negative inversion)
- a. If we were to surrender, ...
- b. Were we to surrender, ... - Subject–auxiliary inversion in condition clause – see English subjunctive: Inversion in condition clauses
The default order in English is subject–verb (SV), but a number of meaning-related differences (such as those illustrated above) motivate the subject and auxiliary verb to invert so that the finite verb precedes the subject; one ends up with auxiliary–subject (Aux-S) order. This type of inversion fails if the finite verb is not an auxiliary:
- a. Fred stayed.
- b. *Stayed Fred? - Inversion impossible here because the verb is NOT an auxiliary verb
(The star * is the symbol used in linguistics to indicate that the example is grammatically unacceptable.)
Subject–verb inversion 
The verb in cases of subject–verb inversion in English is not required to be an auxiliary verb; it is, rather, a full verb or a form of the copula be. If the sentence has an auxiliary verb, the subject is placed after the auxiliary and the main verb. For example:
- a. A unicorn will come into the room.
- b. Into the room will come a unicorn.
Since this type of inversion generally places the focus on the subject, the subject is likely to be a full noun or noun phrase rather than a pronoun. Third-person personal pronouns are especially unlikely to be found as the subject in this construction. For example:
- a. Down the stairs came the dog. - Noun subject
- b. ? Down the stairs came it. - Third-person personal pronoun as subject; unlikely unless it has special significance and is stressed
- c. Down the stairs came I. - First-person personal pronoun as subject; more likely, though still I would require stress
There are a number of types of subject-verb inversion in English: locative inversion, directive inversion, copular inversion, and quotative inversion. See the article on subject-verb inversion.
Inversion in other languages 
Certain other languages, in particular other Germanic languages and Romance languages, use inversion in broadly similar ways to English, such as in question formation. The restriction of inversion to auxiliary verbs does not generally apply in these languages; subjects can be inverted with any type of verb, although particular languages have their own rules and restrictions.
For example, in French, tu aimes le chocolat is a declarative sentence meaning "you like chocolate". When the order of the subject tu ("you") and the verb aimes ("like") is switched, a question is produced: aimes-tu le chocolat? ("do you like chocolate?"). In German, similarly, du magst means "you like", whereas magst du can mean "do you like?".
In languages with V2 word order, such as German, inversion can occur as a consequence of the requirement that the verb appear as the second constituent in a declarative sentence. Thus if another element (such as an adverbial phrase or clause) introduces the sentence, the verb must come next, followed by the subject. An example is: Ein Jahr nach dem Autounfall sieht er wirklich gut aus, literally "A year after the car accident, looks he really good". The same occurs in some other West Germanic languages, like Dutch, where the previous sentence would be Een jaar na het auto-ongeval ziet hij er werkelijk goed uit. (In such languages, inversion can function as a test for syntactic constituency, since precisely one constituent may surface preverbally.)
In languages with free word order, inversion of subject and verb or of other elements of a clause can occur more freely, often for pragmatic reasons rather than as part of a specific grammatical construction.
Theoretical analyses 
Syntactic inversion has played an important role in the history of linguistic theory because of the way it interacts with question formation and topic and focus constructions. The particular analysis of inversion can vary greatly depending on the theory of syntax that one pursues. One prominent type of analysis is in terms of movement in transformational phrase structure grammars. Since these grammars tend to assume layered structures that acknowledge a finite verb phrase (VP) constituent, they need movement to overcome what would otherwise be a discontinuity. In dependency grammars in contrast, sentence structure is less layered (in part because a finite VP constituent is absent), which means that simple cases of inversion do not involve a discontinuity; the dependent simply appears on the other side of its head. These two competing analyses are illustrated with the following trees:
The two trees on the left illustrate the movement analysis of subject-auxiliary inversion in a constituency-based theory; a BPS-style (bare phrase structure) representational format is employed, where the words themselves are used as labels for the nodes in the tree. The finite verb will is seen moving out of its base position into a derived position at the front of the clause. The trees on the right show the contrasting dependency-based analysis. The flatter structure, which lacks a finite VP constituent, does not necessitate an analysis in terms of movement, but rather the dependent Fred simply appears on the other side of its head Will.
- The use of terminology here, i.e. subject-auxiliary inversion and subject–verb inversion, follows Greenbaum and Quirk (1990:410).
- The movement analysis of subject-auxiliary inversion is pursued, for instance, by Ouhalla (1994:62ff.), Culicover (1997:337f.), Radford (1988: 411ff., 2004: 123ff).
- Concerning the dependency grammar analysis of inversion, see Groß and Osborne (2009: 64-66).
- Culicover, P. 1997. Principles and parameters: An introduction to syntactic theory. Oxford, UK: Oxford University Press.
- Greenbaum, S. and R. Quirk. 1990. A student's grammar of the English language. Harlow, Essex, England: Longman.
- Groß, T. and T. Osborne 2009. Toward a practical dependency grammar theory of discontinuities. SKY Journal of Linguistics 22, 43-90.
- Ouhalla, J. 1994. Transformational grammar: From rules to principles and parametrs. London: Edward Arnold.
- Quirk, R. S. Greenbaum, G. Leech, and J. Svartvik. 1979. A grammar of contemporary English. London: Longman.
- Radford, A. 1988. Transformational Grammar: A first course. Cambridge, UK: Cambridge University Press.
- Radford, A. 2005. English syntax: An introduction.Cambridge University Press.