Third Conditional in English

This type of conditional sentence refers to the past. The condition represented by the conditional clause cannot be fulfilled any more. The speaker either regrets not having done something or having done something else. What actually happened was contrary to the condition expressed in the conditional clause, e.g.
If I had known about the meeting, I would have attended it. (but I didn't know and consequently I didn't attend it)
If you hadn't spoken to him about it, you would have saved yourself a lot of trouble. (but you did speak to him and consequently you had a lot of trouble)


Third conditional is used when we imagine the consequences of something that has not come true in the past.


  'If' -clause: past perfect main clause: 'would have /should have'
be: If I had been taller, I would have become a policeman.
have: If he had had any money, he would have left home.
past perfect: If we had gone by car, we would have saved time.
past perfect progressive: If I had been trying harder, I would have succeeded.
could have; If I could have stopped, there wouldn't have been an accident.

Variation 1: 'If' + past perfect + modal

'If' -clause: past perfect tense main clause: modal
If he had known the facts, he could have told us what to do.
If he could have got the facts, he might have told us what to do.
If he had been here yesterday, he could have told us. ability
If he had been here yesterday, he might have told us. possibility
If he had received a present, he should have thanked her. duty