Inversion

Inversion just means putting the verb before the subject as a question:

You are tired ----- Are you tired?

However, we also use inversion when we want to emphasise an event because it is new, rare, unexpected, sudden, impossible, etc 

For example...

1: When we use a negative adverb or adverb phrase at the beginning of the sentence:

I have seldom seen such beautiful work ----- Seldom have I seen such beautiful work. ('Seldom' is at the beginning, so we use inversion. This sentence emphasizes what beautiful work it is.)

Hardly	Hardly had I got into bed when the telephone rang.
Never	Never had she seen such a beautiful sight before.
Seldom	Seldom do we see such an amazing display of dance.
Rarely	Rarely will you hear such beautiful music.
Only then	Only then did I understand why the tragedy had happened.
Not only ... but	Not only does he love chocolate and sweets but he also smokes.
No sooner	No sooner had we arrived home than the police rang the doorbell.
Scarcely	Scarcely had I got off the bus when it crashed into the back of a car.
Only later	Only later did she really think about the situation.
Nowhere	Nowhere have I ever had such bad service.
Little	Little did he know!
Only in this way	Only in this way could John earn enough money to survive.
In no way	In no way do I agree with what you're saying.
On no account	On no account should you do anything without asking me first.

2: We can use inversion instead of 'if' in conditionals with 'had' 'were' and 'should'.

If I had been there, this problem wouldn't have happened ----- Had I been there, this problem wouldn't have happened.

3: We can use inversion if we put an adverbial expression of place at the beginning on the sentence.

All the money we had lost was on the table -----On the table was all the money we had lost.