Inversion

👉 Normally the sentence looks like this:
Subject (who/what) + verb.

• I have never seen such a thing.

👉 In inversion we change the order:
Adverbial / emphasizing word + operator (have/should/will etc.) + subject + rest of the sentence.

Most common cases:

• Never
  Normally: I have never seen such a thing.
  Inversion: Never have I seen such a thing.

• Rarely / Seldom / Hardly ever
  Normal: I rarely go out at night.
  Inversion: Rarely do I go out at night.

• Hardly… when / No sooner… than
  Normally: I had hardly sat down when the phone rang.
  Inversion: Hardly had I sat down when the phone rang.

• Only if / Only after / Only when / Only by
  Normally: You can understand it only if you read carefully.
  Inversion: Only if you read carefully can you understand it.

Formal constructions (Cleft sentences)

👉 Sometimes we want to emphasizeone part of the sentence. Then we use:

It-cleft (It was...)

Structure:
It is/was + underlined element + who/that + rest of the sentence.

• Normally: John broke the window.

• Design: It was John who broke the window.

What-cleft (To, co...)

Structure:
What + sentence + is/was + underlined item.

• Normal: I need a cup of coffee.

• Design: What I need is a cup of coffee.