I have to work on sunday.

我星期天得上班。

A: Can we meet for lunch on sunday?

A: 星期天我们能一起吃午饭吗？

B: No, I'm sorry.  I can't.  I have to work on sunday.

B: 对不起，不行。我星期天得上班。

A: Oh, that's a pity.  I forgot you sometimes work at weekends.

A: 哦，太遗憾了。我忘了你有时周末加班。

B: We could go for lunch on saturday, if you'd like.

B: 周六我们可以一起吃午饭，如果你愿意的话。

A: Yes, that'd be good.  Let's do saturday instead.

A: 好，可以。那我们就改在周六了。

Notes 注释1. When someone asks you a question beginning Can you . . ? / Can you . . ?, the simple response is usually Yes, I can / Yes, I can or No, I can't / No, I can't.
当有人问你以Can you . . ? / 你能 . . 吗?开头的问题时，简单的回答是Yes, I can /是的，我可以。或者No, I can't /不，我不行。

2. Remember various phrases to describe times, eg at the weekend / at the weekend: another day / another day: during the week / during the week: on tuesdays / on tuesdays.
记住用不同的短语来描述不同的时间段。例如at the weekend /在周末；another day /另一天：during the week /在这周内: on Tuesdays /在周二。

3. If you are talking about a possibility, you can use could / could, eg We could go for lunch / We could go for lunch: It could rain tomorrow / It could rain tomorrow.
如果你在谈论某种可能性，你可以用could / 可能。例如We could go for lunch /我们可以一起吃午饭。It could rain tomorrow /明天可能会下雨