We can use the Third Conditional to talk about 'impossible' conditions, impossible because they are in the past and we cannot change what has happened.
- If I had worked harder at school, I would have got better grades.
- If I had had time, I would have gone to see him. But I didn't have time.
- If we had bought that house, we would have had to rebuild the kitchen.
- If we had caught the earlier train, we would have got there on time but we were late.
Notice that the main clause can contain 'would' 'could' or 'might.
- If I had seen him at the meeting, I would have asked him. (But he wasn't there so I didn't.)
- If I had seen him at the meeting, I could have asked him. ( But he wasn't there so it wasn't possible.)
- If I had seen him at the meeting, I might have asked him. (But I'm not sure. Perhaps if the opportunity had arisen.)
- If I had paid more attention in class, I would have understood the lesson.
Also notice that sometimes the 'if clause' is implied rather than spoken.
- I'd have done it. ("if you had asked me but you didn't.")
- I wouldn't have said that. ("if I'd been there.")
- He wouldn't have let him get away with that. ("if he had tried that with me.")
You will find different grammar exercises and more grammar explanations on our sister site English Grammar Secrets