AN UNREAL POSSIBILITY IN THE PAST
IF + PAST PERFECT , WOULD + HAVE + PAST PARTICIPLE
The third conditional is used to talk about an unreal possibility in the past. It means that this conditional should be used when you want to speculate about an action in the past that didn’t happen and so the result of that action is purely speculative.
If I had studied more, I would have passed the test.
If she called the police earlier, she might have prevented the burglary.
If I had not been driving a car that day, I could have avoided the car accident.
If they had read that book, they may have made better choices regarding their finances.