Form
If + S + Past Perfect, S + would/wouldn’t have + V3.
If I had studied harder, I would have passed the exam.
We wouldn’t have missed the flight if we had left earlier.
Meanings
Imaginary Past Situations and Results
We use the Third Conditional to imagine a different past. The situation in the ‘if’ clause did not happen, and so the result in the main clause did not happen either.
If she had saved her money, she would have bought a car.
(Reality: She did not save her money, so she did not buy a car.)
Because it deals with different past actions and results, it’s often used to express regret.
I’m so tired. I wouldn’t have been so tired if I had gone to bed earlier last night.