Use if with had and a past participle, followed by would have and a past participle, to imagine a different result in a finished past situation.
Practical examples
If we had left earlier, we would have caught the train.
لو غادرنا أبكر، للحقنا بالقطار.
I would have helped if you had asked.
لكنت ساعدتك لو طلبت.
If she had seen the email, she would have replied.
لو رأت الرسالة، لردت عليها.