could 也可以表示请求，语气较委婉，主要用于疑问句，不用于肯定句，回答应用can
--- Could I come to see you tomorrow?
--- Yes, you can. / No, I’m afraid not. 5、can 的一些其他惯用法
(1). cannot but, cannot help but, cannot choose but +动词原形， 意思为“不得不，只得”。
a. I cannot help but tell her the truth.
2. cannot help doing sth a. I cannot help laughing.
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3. cannot … too / enough, can never too
a. You cannot be too careful when you cross the street.
b. I cannot thank you too much for your kindness; I owe my progress to you.
二、may, might
1、表示许可。 a. You may go now.
b. We may keep the books for two weeks.
c. --- May I smoke here?
--- Yes, you may. / No, you may not. / No, you mustn’t.
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2、表示可能性
a. He may be waiting for you at the station.
b. They may have got lost. Note:
表示可能性时，may 不用于疑问句。在疑问句中表示“可能”时，通常用“be likely to ” 或“Do you think”这一结构。
3、用于让步状语从句中
a. However hard you may study, you cannot master English in a month. b. Don’t give up whatever difficulties you may meet with.
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4、用于祈使句，表示祝愿。 a. May you succeed!
b. May we never forget each other! 5、might常用于表示委婉的请求或轻微的责备。
a. You might post this letter for me if you are going near a post box.
b. You might have let me know before!
６、用于某些习惯用法中
(1) may /might as well = had better a. It’s getting darker and darker. You may/might as well go home.
(2) may well + 动词原形，意思为
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