The Past Unreal Conditional is used to talk about imaginary or unreal situations in the past. You are describing an action(s) you would have done differently or how an action could have happened differently. (Look at example 1)
Remember Past Unreal Conditional is in the past, so you no longer do the actions in the present.
Only the word "if" is used with the Past Unreal Conditional. When you are discussing imaginary situations "when" cannot be used.
1. If I had been president, we would not be in trouble now.
2. If I would have played the lottery last week, I could have won $100 million.
3. If I had been faster, I could have been an olympian.