We use the third/unreal conditional in the following ways:
1 to talk about unreal past possibilities:
If the weather had been nice yesterday, we would have gone swimming. (but the weather was not nice)
2 to express regrets about the past:
If I hadn’t missed the bus, I would have arrived early. (I regret missing the bus)
We make the third/unreal conditional using this structure:
if + subject + past perfect, subject + would have
We can also use could have or might have instead of would have in the main clause:
If I had had my cell phone with me, I could have called the police.
If the dog hadn’t barked, the thief might have escaped.
Talk to your friends about things in the past you would have done differently.