1.没关系!
Never mind.(这个比较常用!) It does not matter.
It's all right.That is all right.
2.如果愿意帮就回答:"My pleasure." "It 's a pleasure." "It's a pleasure to help you." "I'd be delighted." "Yes,I'll be glad to." "I'd love to very much!" "Yes,of course.I'd be pleased to."
如果拒绝就是:"I would like to help you,but..." "Sorry,I can't manage to do it."
"I am (really) sorry."
3.不用谢!不客气!
Don't mention it.You are welcome.That's all right.Not at all.No thanks.