Inversion for emphasis: Auxiliary–subject order after fronted expressions

Inversion for emphasis places an auxiliary verb before the subject after certain negative or restrictive expressions at the beginning of a clause. The pattern is formal and gives the opening expression strong prominence.

Examples

Never have I seen such a crowded station.

Never is fronted, so have comes before the subject.
Rarely does the museum stay open this late.

Present-simple inversion requires does.
Only after the meeting did we understand the problem.

The restrictive opening triggers inversion in the main clause.

Inverted vs. ordinary word order

Ordinary order

I had never heard that explanation before.

The negative adverb remains inside the clause.

Inversion

Never had I heard that explanation before.

Fronting never creates formal emphasis and reverses auxiliary–subject order.

With ordinary order, the subject comes before the auxiliary. After a fronted negative expression, the auxiliary comes before the subject.

Not every use of only triggers inversion

Inversion follows when only introduces an adverbial expression such as a time, condition or method. It does not normally occur when only directly modifies the subject.

Only then did I notice the error.

Only then is a fronted time expression, so inversion follows.
Only Maya noticed the error.

Only modifies the subject Maya, so normal subject–verb order remains.