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