在表示“必须”时，must与have to的用法比较
1、must着重说明说话人的许可和主观看法，即主观的必要；have to 则表示按客观情况应当或外界条件迫使“不得不”，即客观必要
He must finish the work by tomorrow. (他自己要求)
He has to do what his father says. (他父亲要求他)
I must stop smoking.（我自己要求）
You've got to go and see the boss. (老板要求你)
2、have to 表示一种道义上或法律上的义务、职责或责任；而must则用于表示一种重要或急迫的事情
I'm afraid you'll have to do this; it's a rule of the college. 恐怕你必须这样做；这是学校的规定。
Now I must go or I shall be late for the meeting. 现在我必须得走，否则开会就迟到了。
3、must用于疑问句时，询问听话人的愿望；而have to没有此用法
Do your homework.--Oh, must I? 做你的功课。--啊，我一定得做吗？
4、must只有现在时，如加上时间状语，可表将来，在间接引语中可表过去；have to可有现在时、过去时、将来时、现在进行时、现在完成时、过去完成时等各种不同的时态
You must hand in your papers tomorrow. 明天你们一定交上论文。
I told them they must be careful. 我告诉他们一定要小心。
He had to leave in a hurry yesterday. 昨天他不得不匆忙离开。
We have had to discuss several of our plans with the chief. 我们不得不与主管讨论了我们的几项计划。
I'm having to read this very carefully. 我得非常仔细地看这材料。
5、在虚拟条件句中，表示“必须”或“义务”时，不用must，而用had to或would have to
If you went abroad, you would have to earn your own living. 如果出国，你就不得不自己谋生。
If they had not brought us the valuable information, we would have had to double our effort in working out the results. 若不是他们带给我们这些有价值的资料，我们就得加倍努力才能得到这些成果。
6、have to 有非限定形式，前面可以用would, should, may等情态动词；must则不能
They were very annoyed at having to wait two hours for a bus. 对于不得不等两个小时的公共汽车，他们非常恼火。（动名词形式）
I regret to have to trouble you again. 很抱歉我不得不再次麻烦你。（不定式形式）
Having to be away all day, he only saw her in the evening. 因为整天不在家，晚上他才能见到她。（现在分词形式）
7、在表示“推断；推测”时，只能用must，不能用have to
If he started at 9 o'clock, he must be there by now.如果他9点出发，现在一定到那里了。
8、其否定式mustn't（一定不要；不要；不能）和don't have to/haven't got to (不必；没有必要)的意义不同
The reference books mustn't be taken out of the library. 不准将参考书拿出图书馆。
You don't have to worry about them any more. 你再也不必为他们发愁了。