The Third Conditional

We make the third conditional by using the past perfect after 'if' and then 'would have' and the past participle in the second part of the sentence:

It talks about the past. It's used to describe a situation that didn't happen, and to imagine the result of this situation.

If she had studied, she would have passed the exam (but, really we know she didn't study and so she didn't pass)
If I hadn't eaten so much, I wouldn't have felt sick (but I did eat a lot, and so I did feel sick).
If we had taken a taxi, we wouldn't have missed the plane
She wouldn't have been tired if she had gone to bed earlier
She would have become a teacher if she had gone to university
He would have been on time for the interview if he had left the house at nine