(1) 表示“必须”
此时可用于肯定句、否定句或疑问句，用于否定句时，mustn't的意思是“一定不要”“不能”，而不表示“不必”。如：
Water must be pure if it is to be drunk. 水如供饮用必须净化。
Must I finish the work tonight? 我必须今晚完成这项工作吗?
Adverbs mustn't be put between the verb and the object. 副词不能放在动词和宾语之间。
must主要表示现在或将来，但在间接引语中也可表过去(=had to)。如：
She asked her boss if she must (had to) work overtime. 她请示上司她是否要加班。
对于must开头的一般疑问句，若作否定回答，可以用needn't，不用 mustn't。如：
“Must I clean all the rooms?” “No, you needn't.” “这些房间我都得清扫吗?”“不必。”
(2) 表示推测
意为“准是”“一定是”，通常只用于肯定句。若用于否定句或疑问句，则用 can 代之。如：
That can't be the only way. There must be other ways of solving the problem. 那不是解决问题的唯一办法，肯定有另外的办法。
Jane's light is on. She must be at home. She can't be out. 简的房间里开着灯。她一定在家，不会出去的。
若要谈论过去的情况，必须用“must+have+过去分词”。如：
You must have mistaken my intention. 你一定是误会了我的意图。
She must have been very young when she got married. 她结婚时一定很年轻(www.yingyuyufa.com)。
He must have come this way; here are his footprints. 他一定是走这条路来的，这里还有他的脚印呢。
(3) 表示“偏偏”
有时must可表示某事发生得不早不迟，就在某个当紧的时候，多指某些不受欢迎的事情的发生。如：
The car must break down just we were starting our holidays. 我们刚刚开始休假，汽车偏偏坏了。
Must you worry her with questions, just when she is busy cooking dinner? 她正忙着做饭你干吗偏偏问她问题?
Just when I was busy, the neighbor must come and chatter. 正当我忙碌的时候，邻居偏偏过来聊天。
有时还可指主观上的偏执或固执，常可译为“偏”“偏要”。如：
After I gave her my advice，she must go and do the opposite. 在我给她出主意之后她偏要反着干。
Must you make so much noise? 你就非得弄出这么大的声音吗?
(4) must 与 have to 的区别
两者都表示“必须”，但must 侧重于说话者的主观看法，认为有必要或有义务去做某事;have to 则重于客观需要，含有“不得不”“被迫”之意。如：
Everyone must keep the law. 人人都要守法。
The last train has gone. We'll have to walk home. 最后一班车已经开了，我们得走回家了。
从时态方面看，must 只有现在时一种形式(在宾语从句中可以表示过去)，而 have to 则有多种时态形式。如：
You will have to show your papers at the gate. 在大门口你得出示证件。
We have had to postpone the meeting, due to the Chairwoman's illness. 由于女主席生病了，我们不得不推迟开会。
另外，have to 可以有分词、动名词、不定式等形式，而 must 则没有这些形式。如：
It's annoying to have to get up early on Sundays. 星期日必须早起，令人恼火。
You should offer to help without my having to ask. 你应该不用我请求就提供帮助。
用于否定句时，must 的否定式 mustn't 意为“一定不要”“不允许”;而have to的否定式don't have to 意为“不必”(=needn't)。比较：
You mustn't tell him about it.你一定不要告诉他这件事。(可能是秘密)
You don't have to tell him about it.你不必告诉他这件事。(他可能知道了)
