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.")