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