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。

That'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？

I understand what you're getting at, but (I'm just not interested/I just can't /it just won't be possible) at the moment.

我了解你所说的事情，只是我现在（不感兴趣/无法去/不可能去）。

或者直呼其名来强调语气

Look, Michael, I'm sorry, but it's just not going to be possible at the moment.

Michael，实在抱歉，但是现在确实不可能。

I'd like to help you but I can't take on any new commitments at the moment, I'm afraid.

我很想帮助你，但现在恐怕无法再增加任何新任务了。

给予Ta信心，鼓励Ta独立完成

Actually, I think this is something you could do really well yourself!

事实上，我觉得您完全可以自己做好！

Why not have a go yourself, and give me a shout if you need help?

要不您先自己做着，在需要帮助时喊我？

打时间差

I need to think it over. Can I get back to you later?

我得好好考虑一下。我稍后告诉你好吗？

I'll think about it and let you know, OK?

让我考虑一下，然后再告诉你，可以吗？

“嫁祸”他人

I'm afraid I'm not familiar with that, but have you asked (X)?

我对这个事情不是很熟诶，你问过某某了吗？

Sorry, but that's not really something I can do. Maybe (X) can help you.

抱歉，我实在做不来这事。也许某某可帮你。

Oh dear, I'm terrible at that sort of thing! What about getting (X) to have a look at it?

哦，亲爱的，我最不擅长做这种事情了！让某某来看一下怎么样？

总之要记得，

不要急于回答“是”或“不”然后再后悔。

考虑清楚后再回答，保持礼貌但要坚决，

而且确保让事情朝自己希望的方向发展。