A must表示由说话人加予的义务：

Mother：You must wipe your feet when you come in．

母亲：你进屋前必须先蹭脚。

have to则表示外界的义务：

Small boy：I have to wipe my feet every time I come in．

小男孩：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．

母亲：你今晚必须穿一条礼服裙衣。你可不能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：你必须用字典。我厌烦给你改拼写错误了。

Doctor：You must cut down on your smoking²．

医生：你必须少吸烟。

2 外界的权威

You have to wear uniform on duty， don't you？

你在值班时必须穿制服，是吗？

You have to train very hard for these big matches，I suppose．

我想，为参加这些大赛你必须刻苦训练。

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．

你必须从天桥过马路。

C 第三人称的例句

这时must主要用于书面指令或指导：

Railway company：Passengers must cross the line by the footbridge．

铁路公告：旅客必须由天桥过马路。

Office manager：Staff must be at their desks by 9∶00。

办公室经理：工作人员必须在9∶00前到达各自的办公桌前。

Regulation：A trailer must have two rear³ lamps．

规章：拖车必须有两盏后灯。

只不过是讲述或评论他人的义务时，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，就连高级职员也要在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．

她必须自己给孩子们做衣服穿。她买不起。

They'll have to send a diver⁵ down to examine the hull⁶．

他们必须派潜水员下水去检查船体。

如果用 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这样做。

但must可用于表示说话人对某种责任、义务是赞成或同意的：

A driver who has knocked someone down must stop．

撞人的司机必须停车。（说话人认为这是司机的责任。）

或表示说话人很强烈地认为应该这样：

Something must be done to stop these accidents．

必须采取某种措施以防止这些事故。

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么重要，常常两种形式都可以用：

Typist：I must／will have to buy a dictionary．

打字员：我必须去买一本字典。

Patient：I must／have to/will have to cut down on my smoking．

患者：我必须减少吸烟。

但在表达习惯时，have to更好：

I have to take two of these pills a day．

我必须每天吃两片这种药。

某事对说话人很急迫，或被说话人认为很重要时，则以用must为好：

I must tell you about a dream I had last night．

我必须把我昨晚做的梦告诉你。

Before we do anything I must find my cheque book．

在我们干任何事之前，我必须先找到我的支票簿。

E 其他例句（各种人称）

You must come and see us some time．

你有空一定来看我们。（这是一种常用的随便的邀请方式。）

The children have to play in the street till their parents come home．

孩子们不得不一直在马路上玩到他们的父母回到家里为止。

This sort of thing must stop！

必须制止这种事！（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．

你必须给你叔叔写信并感谢他给你的很好的礼物。

If there are no taxis we'll have to walk．

如果没有出租车了，我们就不得不步行。

If your father was a poor man you'd have to work．

如果你父亲是个穷人，你就不得不工作了。

We have to walk our dog twice a day．

我们每天得遛两次狗。

注意如下常见的例子：

In shop window：Closing down sale！Everything must go！

商店橱窗里的告白：关门大甩卖！卖完为止！

F 表示过去的义务的肯定句用 had to表示过去的义务时，而且说话人本身的权威和外界的权威无法区分时，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，不得不向汤姆借钱。

You had to pay duty on that，I suppose？

我想你不得不为那个付了税吧？

There were no buses so he had to walk．

没有公共汽车，所以他只好走着去。

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