Past Unreal Conditional

The Past Unreal Conditional is used to talk about imaginary situations in the past. It is used to imagine different outcomes due to different decisions or actions in different circumstances:
If I had gone out less, I would probably have studied more.
I would have travelled more if I had had more time.
If I had studied French at school, I would have got the job.
Danny would have got the promotion if he hadn't insulted the boss's wife.

With the Past Unreal Conditional only 'if' can be used when the situations discussed are imaginary. 'When' cannot be used.

Conditionals and modals

There are some special conditional forms for modal verbs in English:
would have + can = could have
would have + shall = should have
would have + may = might have
The words "can," "shall" and "may" cannot be used with "would have." Instead, they must be used in these special forms:
If I had gone to England I could have studied interior design.
If he had time, he might have come to meet us.
When Sarah asked me to come to the party, I should have gone.