在这种用法中must表示出于主观意识而必须要做某事；have to却表示由于某种客观原因而不得不做某事。除此以外，have to自身有时态的变化形式。如：
I have to go now for I've got a meeting 15 minutes later.我现在必须要走了，因为我15分钟后有个会要开。
He has to be back home by five to fetch his son from kindergarten .
他必须在五点钟前回家去幼儿园接他的儿子。
Jenny had to tidy up the room before her mother came back .
珍尼不得不在她妈妈回家以前将房间整理好。
We'll have to reconsider the whole thing .
我们将不得不重新考虑整个事情。
注意：在这种用法中，回答以must引导的问句时，若是否定答复，不可以用mustn't 而需要用needn't 或是don't have to，因为mustn't意思是"绝不能、一定不要"，而没有"不必"的意思。例如：
--Must we hand in our exercises today ? 我们今天必须交作业吗？
--Yes , you must .是的，必须交。
--No , you needn't (or , you don't have to ).
不，不必今天交。
3）表示禁止（用于否定句）
You must not speak ill of others .
你一定不要说别人的坏话。
Cars must not parked here . 这里禁止停车。
Smoking must not allowed in the office .严禁在办公室吸烟。
You mustn't talk to girls like that .你绝不能那样对女孩子说话。
4）表示猜测、推测must常用于肯定句中表示猜测。对现在或未来的事进行猜测时，后接动词原形；对已经发生的事进行猜测时，后接have + 过去分词。例如：
You must be tired after working so long .你工作这么久以后，肯定累了吧。
It must be that naughty boy crying outside .肯定是那个调皮的孩子在外面大叫。
The idea must be accepted by the society sooner or later.
这种想法肯定或迟或早会被社会所接受的。
It must have rained last night for it is so wet outside .
外面这么湿，昨晚肯定下雨了。
I didn't see you in class yesterday. You must have been
absent .我昨天上课没见着你，你肯定没有来。
Ought （后接带to的不定式构成谓语动词）应该
1）表示出于职责、义务该做某事
You ought not to be so careless in your work .你不应该在工作上如此粗心大意。
We ought to study hard for the sake of our motherland.
我们应该为了我们的祖国而努力学习。
His doctor said to him that he oughtn't to smoke so much .
他的大夫说他不应该抽这么多烟。
Oughtn't you to give us a chance to try ?
你难道不应该给我们一个尝试的机会吗？
2) 表示极有可能发生某事
As an auto repairman , Dick ought to know what is wrong with the car .
狄克作为一个汽车修理工总该知道这部车的毛病所在吧。
If we set off right now , we ought to be able to get there in time .
如果我们马上出发，我们应当能够按时到达那里。
Tomorrow ought to be a nice day for we have such a fine sunset today .
今天有这么好的晚霞，明天准是个晴天。
need（用于否定句和疑问句）需要
There are still one and a half hours to go , we needn't be insuch a hurry .
还有一个半小时，我们无需如此慌慌张张。
I hardly need say how much we missed you .不用说我们是多么想念你啊。
Need you go so soon ?你需要这么早就去吗？
You need have no anxiety on my account .你不必为我而着急。
dare , dared（用于否定句、疑问句和疑问句）敢
Dare you stay here alone in the night ? 你敢在夜里呆在这儿吗？
Nobody dared mention that matter. 没有人敢提那件事。
How dared you say that to her ? 你怎么敢对她说那件事？
Even if you dare do it , I won't allow you to because it's too dangerous .
即使你敢做，我也会让你做那件事，因为太危险了
shall 用作情态动词时,适用于第二人称和第三人称。
主要表示下面几层意思：
1）许诺
You shall hear everything directly you come .你一来就可以听到一切了。
"Whatever you want you shall have , "said the Fairy .
仙女说："你想得到什么，你就可以有什么。"
I don't want to be hard on your daughter ; she shan't be pressed .
我不想对你女儿太严厉，她不会太受压的。
I promise that you shall see her again before long .
我保证你不久就能再见到她。
2）命令
You shall come to my office immediately .你必须马上来我的办公室。
She shall not stay in my garden .她不可以呆在我的花园里。
He shall not come into my study . 不许他进我的书房。
You shall do as you are told .按告诉你的那么做。
3）决心（表示某事一定会发生）
That day shall come .那一天一定会到来。
It has been decided that he shall be given the job .
肯定给他那份工作的事已经定下来了。
This law shall come into effect on January 1st 2001
这个法律于2001年元月1日生效。
4）规定
Each competitor shall wear a number .每个参赛者要佩戴一个数字标。
The renter shall keep the house in a neat and orderly state .
租赁者要保持房屋整洁有序。
The supplier shall make shipment before October and the purchaser shall make payment within 15 days after
receipt of the goods .卖方十月以前装运，买方在收到货物后十五天内付款。
