Imaginary situations in the past with if‏

Past Conditionals
Past Unreal Conditional
FORM
[If ... Past Perfect ..., ... would have + past participle ... ]
[... would have + past participle ... if ... Past Perfect ...]
USE
The PastUnreal Conditional is used to talk about imaginary situations in the past. You can describe what you would have done differently or how something could have happened differently if circumstances had beendifferent.
Examples:

If I had owned a car, I would have driven to work. But I didn't own one, so I took the bus.
She would have traveled around the world if she had had more money. But she didn'thave much money, so she never traveled.
I would have read more as a child if I hadn't watched so much TV. Unfortunately, I did watch a lot of TV, so I never read for entertainment.
Mary wouldhave gotten the job and moved to Japan if she had studied Japanese in school instead of French.
If Jack had worked harder, he would have earned more money. Unfortunately, he was lazy and he didn't earnmuch.
A: What would you have done if you had won the lottery last week?
B: I would have bought a house.
A: What city would you have chosen if you had decided to move to the United States?
B: Iwould have chosen Seattle.

IMPORTANT Only use "If"
Only the word "if" is used with the Past Unreal Conditional because you are discussing imaginary situations. "When" cannot be used.
Examples:I would have bought that computer when it had been cheaper. Not Correct
I would have bought that computer if it had been cheaper. Correct

EXCEPTION Conditional with Modal Verbs
There are somespecial 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 usedwith "would have." Instead, they must be used in these special forms.
Examples:

If I had gone to Egypt, I could have learned Arabic.
If she had had time, she might have gone to the party....