Inversion for emphasis

After a negative or limiting word at the start of a sentence, formal English inverts the subject and the auxiliary, as in a question: never have I seen such a crowd, rarely does she complain, not only did he win, but he also broke the record.

With no auxiliary, add do, does or did: seldom do we meet. Other triggers include hardly, no sooner, little, under no circumstances and only then: hardly had we arrived when it started to rain.

Conditionals can drop if and invert too: had I known means if I had known, and should you need help means if you need help.