Third Conditional: no possibility
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.
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.
Notice that we are thinking about an impossible past condition. You did not win the lottery. So the condition was not true, and that particular condition can never be true because it is finished. We use the past perfect tense to talk about the impossible past condition. We use WOULD HAVE + past participle to talk about the impossible past result. The important thing about the third conditional is that both the condition and result are impossible now.
Sometimes, we use should have, could have, might have instead of would have, for example: If you had bought a lottery ticket, you might have won.
Look at some more examples in the tables below:
IF condition result
past perfect WOULD HAVE + past participle
If I had seen Mary I would have told her.
If Tara had been free yesterday I would have invited her.
If they had not passed their exam their teacher would have been sad.
If it had rained yesterday would you have stayed at home?
If it had rained yesterday what would you have done?
result IF condition
WOULD HAVE + past participle past perfect
I would have told Mary if I had seen her.
I would have invited Tara if she had been free yesterday.
Their teacher would have been sad if they had not passed their exam.
Would you have stayed at home if it had rained yesterday?
What would you have done if it had rained yesterday?
http://www.englishclub.com/grammar/v...ditional_4.htm
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