A must表示由说话人加予的义务：
Mother：You must wipe your feet when you come in．
妈妈：你进屋前需要先蹭脚。
have to则表示外面的义务：
Small boy：I have to wipe my feet every time I come in．
小孩子：我每次进屋前都得先蹭掉鞋上的土。
B 第二人称的例句
1 说话人的权威
Mother：You must wear a dress tonight．You can't go to the opera in those dreadful1 jeans．
妈妈：你今晚需要穿一条礼服裙衣。你可不可以穿着那些难看的牛仔服去歌剧院。
Employer：You must use a dictionary．I'm tired of correcting your spelling mistakes．
雇主：你需要用字典。我厌烦给你改拼写错误了。
Doctor：You must cut down on your smoking2．
大夫：你需要少抽烟。
2 外面的权威
You have to wear uniform on duty， don't you？
你在值班时需要穿制服，是吗？
You have to train very hard for these big matches，I suppose．
我想，为参加这类大赛你需要刻苦练习。
You'll have to get up earlier when you start work，won't you？
你开始上班时，就要早起一些了，是吗？
You'll have to cross the line by the footbridge．
你需要从天桥过马路。
C 第三人称的例句
这个时候must主要用于书面指令或指导：
Railway company：Passengers must cross the line by the footbridge．
铁路通知：旅客需要由天桥过马路。
Office manager：Staff must be at their desks by 9∶00。
办公室经理：员工需要在9∶00前到达各自的办公桌前。
Regulation：A trailer must have two rear3 lamps．
规章：拖车需要有两盏后灯。
只是讲述或评论别人的义务时，则用 have to：
In this office even the senior4 staff have to be at their desks by 9∶00．
在这个办公室，就连高级员工也要在9∶00前到达他们各自的办公桌前。
She has to make her children's clothes．She can't afford to buy them．
她需要自己给孩子们做衣服穿。她买不起。
They'll have to send a per5 down to examine the hull6．
他们需要派潜水员下水去检查船体。
假如用 must代替上面例子中的 have to，就可能意指说话的人有权下命令如此做。
但must可用于表示说话人对某种责任、义务是赞成或赞同的：
A driver who has knocked someone down must sTOP．
撞人的司机需要停车。（说话人觉得这是司机的责任。）
或表示说话人非常强烈地觉得应该如此：
Something must be done to sTOP these accidents．
需要采取某种手段以预防这类事故。
D 第一人称的例子
第一人称为主语时，must和 have to二者之间的差别不那样要紧，常常两种形式都可以用：
Typist：I must／will have to buy a dictionary．
打字员：我需要去买一本字典。
Patient：I must／have to/will have to cut down on my smoking．
病人：我需要降低抽烟。
但在表达习惯时，have to更好：
I have to take two of these pills a day．
我需要天天吃两片这种药。
某事对说话人非常急迫，或被说话人觉得非常重要时，则以用must为好：
I must tell you about a dream I had last night．
我需要把我昨晚做的梦对你说。
Before we do anything I must find my cheque book．
在大家干任何事之前，我需要先找到我的支票簿。
E 其他例句（各种人称）
You must come and see us some time．
你有空肯定来看大家。（这是一种常见的随意的邀请方法。）
The children have to play in the street till their parents come home．
孩子们不能不一直在马路上玩到他们的爸爸妈妈回到家为止。
This sort of thing must sTOP！
需要制止这种事！（说话人或者有权或者对此事感觉强烈。）
You must write to your uncle and thank him for his nice present．
你需要给你叔叔写信并感谢他给你的非常不错的礼物。
If there are no taxis we'll have to walk．
假如没出租车了，大家就不能不步行。
If your father was a poor man you'd have to work．
假如你爸爸是个没钱人，你就不能不工作了。
We have to walk our dog twice a day．
大家天天得遛两次狗。
注意如下容易见到的例子：
In shop window：Closing down sale！Everything must go！
商店橱窗里的表白：关门大甩卖！卖完为止！
F 表示过去的义务的一定句用 had to表示过去的义务时，而且说话人本身的权威和外面的权威没办法区别时，只可用一个形式，即had to：
I ran out of money and had to borrow from Tom．
我没钱花了，不能不向汤姆借钱。
You had to pay duty on that，I suppose？
我想你不能不为那个付了税吧？
There were no buses so he had to walk．
没公共汽车，所以他只好走着去。
