This is a practical oversimplification which will be right most of the time. For more detailed information see Mastery of the Tenses!
"would have done"
For past actions that were not possible for some reason.
- She would have studied but she didn't have any time.
- He would have come but he was working.
Used in the Consequence clause when it was improbable or impossible. NOT IF CLAUSE.
In Third Conditional sentences
- If we had studied, we would have passed the exam.
- If he hadn't snowed, we would never have met.
- They would have won if he had played!