Inversion is when we put an auxiliary verb before the subject of a clause. We use do/dies/did if there is no auxiliary.
I never saw such a wonderful sight again.
auxiliary + subject + clause
Never again did I see such a wonderful sight.

Inversion is common in written formal texts, but it is also used in informal spoken English to add dramatic effect or emphasis.
No way would I ever go on a trip like that! (There is no way that I would ever go on trip like that)
Not in a million years would I agree to cross Asia on a motorbike!

negative adverbials
In formal English, it is common to use inversion after negative adverbial expressions and restrictive words such as only, never, hardly and little.
At no time did they stop to think about the consequences.
Not until the ambulance arrived did we realise how serious it was.
Hardly had I got through the door when the phone rang.
Never before/Rarely/Seldom had I seen such landscapes.
Little did we know that he had followed us.
No sooner had he got the job than he asked for a pay rise.
No longer will we accept these poor conditions.
Only then did they see what a fantastic chance they'd been offered.
Only now/after all these years has the crime been solved.
Not since the 80th has she written such a superb novel.
Not only do they want a pay increase, they (also) want reduced hours.
Under no circumstances/On no account should you leave your child unattended.
Scarcely had they left the room than people started talking about them.

conditional clauses
In formal English, inversion can be used with conditional sentences.
If we had known how much it was going to cost, we would never have chosen it. = Had we known how much it was going to cost, we would never have chosen it.
If you were to have approached from the other direction, you might have seen the signs. = Were you to have approached from the other direction, you might have seen the signs.

Fronting comparative or superlative phrases:
so, such, also, as, though
So terrible was the storm that the ship sank.
Such was his financial position that his friends started to worry.
Try as they might, they could not win the race.
Exhausted though he was, he stumbled on.