你要做的事情就是：

1)说出感谢

2)说“不”

3)给出理由

还有可能：

4)建议其他时间或日期

举个例子

Thank you so much for the invitation, but I'm afraid I have a prior engagement.

非常感谢您的邀请，但我已经有约了。

更正式一些

Thanks for the invitation!I'm really sorry, I can't come - I've already got other plans.

感谢您的邀请！不过很抱歉我不能来，因为我已计划了其他事情。

还可以说

I'd love to, but I'm afraid I can't.Would another time be possible?

我很想去，不过恐怕不行。可以换个时间吗？

I'm very sorry, but I won't be able to make it.Another time would be great, though!

很抱歉，我无法去。要是换个时间就太好了！

It's a lovely idea, but I won't be able to join you this time.

好主意，但我这次无法参加了。 T

hat's very kind of you, but …

您太好了，不过...

Could I join you later?I just have to finish something.

我可以稍后加入你们吗？我必须先完成一些事情。

Today's not so good - how about tomorrow?

今天不太合适，明天如何？