Inversion

After certain negative or limiting expressions at the beginning of a sentence, we put the auxiliary before the subject, like in a question.

Negative Adverbials

After never, rarely, seldom, hardly, little, not only, no sooner at the front, use inversion.

Never have I seen such a mess. — not Never I have seen such a mess.
Rarely does she go out.
No sooner had I left than it started.

Conditional Inversion

We can leave out if and invert with had, were, should.

Had I known, I would have helped.
Were I you, I would apologise.
Should you need help, ask me.

Other Cases

Only after lunch did we start.
Under no circumstances should you sign.

Quick Reference

Negative adverbial + auxiliary + subject
Little did he know.
Conditional: Had/Were/Should + subject
Had I known...