①One cannot be too careful.

　　②You cannot work too hard.

　　③I cannot thank you enough.

may (might)

　　1)表允许，might语气更委婉。

　　①You may take whatever you like.

　　②May /Might I ask for a photo of your baby?

　　 (在回答以may引起的问句时，用Yes, please. / Certainly. / Please don't. / You'd better not. / No, you mustn't.等，以免显得不客气否定回答为Please don't.或No, you mustn't)I

　　③ ---May we leave now?

---No, you mustn't. You haven't finished your homework yet.

2) 表可能性（事实上）。might不表示过去，而是表示不确定性。 may not 可能不

① He may/might be at home.

②She may/might not know about it.

③She may be staying at home.

3)表祝愿

　　①May your friendship last forever.

　　②May you have a happy new year.

4)may/might as well + do "最好，倒不如 "

　　①You may as well stay here over the night.

　　②This holiday isn't much fun. We might as well be back home.

5) may / might (not) have done ◆本能够做而没能做//本不能够做而做了

　　　　　　　　　　　　　　◆表示推测过去某事"也许"发生了.may 比 might 表示的可能性稍大些。

　　I can't find my keys. I may / might have left them at the school yesterday.

must 没有过去式，表示现在或将来

1) 表示义务。意为"必须"（主观意志）。

①We must do everything step by step.

　　②--Must we hand in our exercise books now?

　--No, you needn't. / No, you don't have to.不必

2）表示推测。只用于肯定句，"想必、准是、一定"，否定推测用can't。

　　①She's wearing a diamond necklace. She must have a lot of money.

　　②It must be raining outside.

　　③It must have rained last night, for the ground is quite wet.

④That can't be the postman---it's only seven o'clock.