Past unreal conditionals are often used to express wishes about the past. They often show regret, or sad feelings about something that happened in the past. Here's an example:

"If I had studied, I would have passed the test".

In this example, there is an implied wish that the speaker had studied.

A conditional sentence has two parts. The first part is the if clause: "If I had studied." The second part is the result clause: "I would have passed the test."

To form the past unreal conditional, use the past perfect in the if clause. Use had followed by a past participle verb to form the past perfect. In the result clause, use would have followed by the past participle.

Use past unreal conditionals to talk about how you would have acted differently if you had had more information. Basically, "if I had known A, I would have done B". Imagine you brought chocolate for a co-worker. You didn't know it at the time, but your co-worker was diabetic. In this situation, you could say, "If I had known you were diabetic, I would not have brought chocolate."