星级典句：

第一句：I'm afraid I can't do that.
恐怕不行。

A: Can you lend me some money?
你能借我点钱吗？
B: I'm afraid I can't do that.
恐怕不行。

第二句：I'm sorry but I can't help you.
抱歉我不能帮你。

A: Could you write the thesis for me?
你能帮我写论文吗？
B: I'm sorry but I can't help you.
抱歉我不能帮你。
A: You know, I'm so busy with my part-time job recently. Would you please...
你知道我最近忙兼职，你能……
B: Sorry. But I can't.
对不起，我不能这么做！

拒绝别人的较为委婉的说法有：

I wish I could help you, but I can't.
我希望我能帮上你，可我不能。

I'm sorry, but I'm busy right now.
对不起，我现在很忙。

而强硬的拒绝可以说：

I said no and I mean it.
我说不行就不行。