Inversion means putting the verb before the subject.
The normal order of words is reversed or inverted, generally to add emphasis or forcefulness, or to give special effect. It also sounds quite formal.
The part we want to emphasise is generally at the beginning of the sentence.

Although inversion of the verb before the subject is a common phenomenon in English sentences, it is seldom used in everyday speech, except in questions and with 'so/neither'.
It is more often found in formal statements or in writing.
In a sentence with no special effect or emphasis, the normal order of words is retained.

Example 1 : I have never seen such a beautiful rose.
Example 2 : Never have I seen such a beautiful rose.

In example n°2 inversion is used to emphasise the fact that in your whole lifetime you have not seen such a beautiful rose.

When can we use inversion?

It is sometimes difficult to remember when inversion is or can be used. Here are some guidelines and examples to help you.

In normal everyday English, inversion is used :

• To make questions : Does he?   Can you?
• After 'so', 'neither', 'nor' :  So do I, neither do I, nor do I. ( more here...)
  

In written English, as well as in a very formal style, inversion can be used in the following cases :

• After negative adverbial expressions  :
  • Under no circumstances can we accept credit cards.
  • Under no circumstances can we allow dogs in the restaurant.
  • In no way can he be held responsible.
  • At no time did she say she would come.
  • At no point was the price mentioned.
  • By no means is it easy to overcome shyness.
  • In no way do I agree with your methods.
  • Never before have I witnessed such violence.
  • Never again will I return to that hotel. It was awful!
  • Not for a moment did I think I would get the job!
  • Not since my childhood have I had so much fun!
  • Not since the beginning of records have they noted such high temperatures.
  • Not until I heard my name did I believe I had won the race.
  • Not until the doorbell rang did I think my father would come.
  • On no account should you climb onto the roof.
  • On no account should you allow children to use these scissors.
    
    
• After adverbial expressions of place :
  • Round the corner came the postman.
  • In the sink sat a stack of dirty dishes.
  • On the doorstep was a bunch of flowers.
  • At the main entrance stood a policeman.
    
    
• After 'seldom', 'rarely', 'never', and 'little':
  • Seldom have I seen such a beautiful view.
  • Rarely did he pay anyone a compliment.
  • Never had I felt so happy.
  • Little did he imagine how dangerous it would be.
    
    
• After 'hardly', 'scarcely', 'barely', 'no sooner', when one thing happens after another.
  • Hardly had I begun to speak when I was interrupted.
  • Scarcely had we started our meal when the phone rang.
  • Barely had they finished the match when the rain started to fall.
  • No sooner had I arrived than they all started to argue.
  
  - Note that hardly, scarcely and barely are followed by when.
  - No sooner is followed by than.
  
  
• After adverbial expressions beginning with 'only' and 'not only'.
  • Only after the meeting did I realize the importance of the subject.
  • Only by working extra hours could they afford to buy a car.
  • Only if the weather improves will the picnic be confirmed.
  • Only when the plane landed safely did he calm down.
  • Not only was the food plentiful, it was also delicious.
  • Not only could she sing, she could also play the piano.
  • Not only was the car slow, it was also very uncomfortable.
    
    
• Conditional structures with inversion :
  In conditional sentences we can sometimes replace the 'if' with an inversion:
  • If I had known it would be so difficult I would never have enrolled.
    With inversion: → Had I known it would be so difficult I would never have enrolled.
  • If we had known she was alone we could have called her.
    With inversion: → Had we known she was alone we would have called her.
    
    
• After exclamations with 'here' and 'there' :
  • Here comes the winner!
  • Here comes the bus!
  • There goes the bell!
  • There goes all our money!
    
    
• After 'so/such' …'that': (formal)
  • 'So + adjective + verb ... 'that'
    - So expensive was the journey that they had to borrow money.
    - So strange were my surroundings that I couldn't sleep.
    - So exhausted was the hiker that he went straight to bed.
    
    
  • 'Such' + verb + noun ... 'that'
    - Such was the wind that we couldn't open the door.
    - Such was their excitement that the children couldn't stay quiet.
    - Such was her fear that she couldn't utter a word.
    
    
• With verbs of reporting such as 'say' or 'ask' in direct speech:
  • "I love you" said Harry.
  • "How far is it?" asked the passenger.