would have + past participle

[haseli22]

Hi everyone,
Merry Christmas. I have a question: When do we use the structure “would have + past participle?” It’s like must have + p.p or should have + p.p, but I don’t know when it is used and how.
Thanks in advance.
Mehdi.

[Flittle]

This is a conditional form and is used in conjunction with an if clause using the past perfect(pluperfect).
If I had left earlier, I would have caught the train.
The two clauses can be changed around in which case the comma is not necessary.
I would have caught the train if I had left earlier.
Frank