(1) must do表示“必要性”。(另见“have to...”条)
- We must work hard so that we can fulfil our task ahead of schedule.
我们必须努力工作以便能提前完成任务。
(2) 在表示“不必要”时，不用mustn't，而用needn't， don't need to或don't have to。
- A: Must I come to the meeting tomorrow?
我明天必须去开会吗？
B: No, you needn't .
不，没必要。 - You needn't work tomorrow if you don't want to.
如果你不愿意，你明天可以不工作。
(3) must do还可以表示“告诫，命令，必须，禁止”(包括对说话者自己)。
- You mustn't be late again!
你一定不要再迟到了! - You really must stop smoking.
你真的不能再吸烟了。 - I must be here by 8 o'clock at the latest.
我最迟必须在8点之前到这儿。
(4) 当所谈及的“必要性”来自说话者以外的其他方面(如规章、某人的命令)时，可以用must,但更常用have to。
- We have to work from 8 o'clock in the morning till 4 o'clock in the afternoon.
我们必须从上午8点一直工作到下午4点。
(5) must只用于现在和将来的“必要性”，指过去的，要用had to。
- I had to leave early that day because I wasn't feeling very well.
那天我必须早点离开，因为我感觉不太舒服。
(6) 在疑问句中，must用于询问对方的意愿或想法。
- Must Mr. Smith come to the meeting?
史密斯先生要来参加会议吗？ - Why must you always leave your dirty clothes in the bathroom?
你为什么总把脏衣服留在浴室里呢？
(7) must do/be的形式用在肯定句中表示对某件事情逻辑上的推定。
- Jane must have some problem: she keeps crying.
简一定是有什么事：她一直在哭。 - It must be 6 o'clock now.
现在肯定有6点了。
(8) 在否定句和疑问句中，这种推定应分别使用can't和can。
- That can't be the postman — it's only seven o'clock.
那不会是邮递员——这才7点。 - What do you think this letter can mean?
你认为这封信会是什么意思？
(9) 对于正在进行的事项的推测，要用must be doing的形式。
- Let's hurry up — they must be anxiously waiting for us.
咱们快些，他们一定在焦急地等我们。
(10) ①对于过去某项事情的推测，要用“must have done”的形式。
- Mary must have gone home — her bike is gone.
玛丽一定回家了——她自行车不在了。
②在疑问句和否定句中，要用can和can't have done的形式。
- I don't think he can have heard you, call again.
我认为他听不见你喊他，再喊一遍吧。 - Where can Mary have put my watch?
玛丽能把我的表放哪儿？
