Subject-auxiliary inversions not associated with questions [duplicate]

Solution 1:

Subject-Auxiliary Inversion with Adverb-Fronting is simply a Negative Polarity Item (NPI).

‘NPI’ is a term applied to lexical items, fixed phrases, or syntactic construction types that demonstrate unusual behavior around negation. NPIs might be words or phrases that occur only in negative-polarity contexts (fathom, in weeks) or have an idiomatic sense in such contexts (not too bright, drink a drop); or they might have a lexical affordance that only functions in such contexts (need/dare (not) reply); or a specific syntactic rule might be sensitive to negation, like Subject-Auxiliary Inversion with Adverb Fronting.

Subject-Auxiliary Inversion is OK with a fronted adverb that is a Negative Trigger. E.g, ever and frequently are not negative, but never is; hence:

• *Ever have I seen such a thing.
• *Frequently have I seen such a thing.
• Never have I seen such a thing.

Details of this and related constructions are available here.

Solution 2:

Yes, there is such a rule, though it is not completely reliable. When a sentence starts with certain words that are both stressed and strongly limiting or negative, inversion is normally applied in (educated) writing, though not universally so. It expresses a certain rhetorical force. In casual writing, I believe it is less common. Usually words and phrases like never, no...ever, rarely, no sooner, only, barely, scarcely, seldom are involved. It is perhaps safer to look up a list of words that have this effect than apply it to a category, if you're not sure, because it doesn't work for all such words.

Never had she met a man like Socrates.

In no place but Athens had she ever known true happiness, and so there she stayed.

In few other places had she ever been happy.

Rarely are such bonds formed.

No sooner had he married Xanthippe than her scourging tongue became manifest.

Only in her did he find an opponent who could wear him down with words.

Not only could she be voluble, but also scathing, when she so chose.

Barely had he left the house when she would scream and call him back to finish chores.

And scarcely had he finished one chore when she would make him do another.

Seldom did he arrive on time.

This inversion probably has something to do with pragmatic focus laid on the limiting or negating phrase; cf. ordinary focal inversion as in this is Octavian, and him I have adopted as my son.

There is also a broad, vague relation between negations and questions in English and other languages that may have something to do with this construction; consider how do-support is needed with questions and negations but not positive statements. But this connection is weak and speculative.