1. 表示“必须”、“一定要”
(1) 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”,而不是“不必”:You must finish it today. 你一定要在今天完成它。You must see the doctor. 你一定要看医生。Must you go so soon? 你一定得这么早就走吗?We mustn't think only of ourselves. 我们可不能只考虑自己。

(2) 主要表示现在或将来,但有时(如在间接引语中)也可表过去(=had to):She asked if she must (had to) leave. 她问她是否一定要离开。

(3) 对于must 开头一般疑问句,若要作否定回答,可用needn't,不用mustn't:"Must I come over tonight?" "No, you needn't." “我今晚必须过来吗?”“不必。”

(4) 其后可接被动结构或进行形式:Something must be done to stop it. 必须采取措施来阻止它。I think I must be going. I think I must be going. 我想我得走了。2. 表示推测。意为“准是”、“一定是”(1) 通常只用于肯定句,在否定句或疑问句用 can 代之:It must be true. 那一定是真的。It must be true. 比较:Can it be true?  那可能是真的吗? / It can't be true. 那不可能是真的。(2) 后接动词原形,表示对现在情况的推测: He must be wrong. 他一定错了。The man must have a lot of money. 这个人一定有不少钱。注:must 表示对现在情况的推测后接动词原形时,该动词通常为状态动词(如 be, have, know 等),若为动作动词,通常要转换其它说法。“他一定会赢”,通常不说He must win,可说He is sure to win。(He must win的意思是“他必须要赢”) (3) 其后可接进行式或完成进行式,用以谈论一个正在进行的动作:He must be writing a letter to his girl friend. 他一定在给他女朋友写信。Someone must have been smoking here.一定有人一直在这里抽烟。(4) 后接不定式的完成式,用来谈论已发生的情况:He must have arrived already.他一定已经到了。I didn't hear the phone. I must have been asleep. 我没有听到电话铃,我一定是睡着了。3. 表示“应当”。相当于should, ought to的意思You must say hello to her.你应当跟她打个招呼。We must go and have a weekend there.我们应当到那里度周末。4. 表示“偏偏”有时表示不巧,有时表示固执,通常都是指令人不快的事:Must you make so much noise? 你就非得弄出这么大的声音吗?Why must you always interrupt me? 你为什么硬是老要打断我?We were ready to leave, but the baby must catch cold.  我们刚准备好要走, 可孩子偏偏得了感冒。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。正当我忙碌的时候,邻居偏偏过来聊天。