The first conditional and second conditionals talk about the future. With the third conditional we talk about the past. We talk about a condition in the past that did not happen. That is why there is no possibility for this condition. The third conditional is also like a dream, but with no possibility of the dream coming true.

 Example:
Last week you bought a lottery ticket. But you did not win

condition	result
Past Perfect	WOULD HAVE + Past Participle
If I had won the lottery	I would have bought a car.

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

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.)

Also notice that sometimes the 'if clause' is implied rather than spoke

• 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.")