I'm so sorry for hurting your arm. It was an accident.
That's okay, I know you didn't do it deliberately.
I'm sorry that I broke your pencil.
That's okay, it was an accident.
I'm very sorry that I forgot to return your umbrella.
Don't worry about it, I have an extra one.
I'm terribly sorry about breaking your vase and spilling the flowers.
I forgive you. Just be more careful next time.