Explain instances in the novel where someone is forgiven. What constitutes true forgiveness? Why is forgiveness so important?

I think Amir is forgiven. It will take him time to realize it but he was always forgiven by Hassan. When Hassan said, "For you a thousand times over", he meant it. Hassan was not angry at Amir over what happened. It was Amir that needed to forgive himself. It isn't until he rescues Sohrab that he begins to forgive himself. When he flies the kite with Sohrab in America we know that there is hope that Amir can forgive himself.