The Past Unreal Conditional explained easily

Why is The Past Unreal Conditional called Unreal?  

It is not called Unreal because it is an unreal situation, it is called Unreal because it is an action that cannot already be done.

Why is it impossible to do this action?

It is not possible to do the action, because the moment when was possible to do it, is over.

Let's see an example:

If I had known, I would have called you.

• In the previous sentence, we can see that the person who talks cannot already call to his/her listener, because he/she didn't get on time some information.

• If the person who talks had got the information, he/she would have called him/her listener.