在這種用法中must表示出于主觀意識而必須要做某事；have to卻表示由于某種客觀原因而不得不做某事。除此以外，have to自身有時態的變化形式。如：
I have to go now for I've got a meeting 15 minutes later.我現在必須要走了，因為我15分鐘后有個會要開。
He has to be back home by five to fetch his son from kindergarten .
他必須在五點鐘前回家去幼兒園接他的兒子。
Jenny had to tidy up the room before her mother came back .
珍尼不得不在她媽媽回家以前將房間整理好。
We'll have to reconsider the whole thing .
我們將不得不重新考慮整個事情。
注意：在這種用法中，回答以must引導的問句時，若是否定答復，不可以用mustn't 而需要用needn't 或是don't have to，因為mustn't意思是"絕不能、一定不要"，而沒有"不必"的意思。例如：
--Must we hand in our exercises today ? 我們今天必須交作業嗎？
--Yes , you must .是的，必須交。
--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 .
你一定不要說別人的壞話。
Cars must not parked here . 這里禁止停車。
Smoking must not allowed in the office .嚴禁在辦公室吸煙。
You mustn't talk to girls like that .你絕不能那樣對女孩子說話。
4）表示猜測、推測must常用于肯定句中表示猜測。對現在或未來的事進行猜測時，后接動詞原形；對已經發生的事進行猜測時，后接have + 過去分詞。例如：
You must be tired after working so long .你工作這么久以后，肯定累了吧。
It must be that naughty boy crying outside .肯定是那個調皮的孩子在外面大叫。
The idea must be accepted by the society sooner or later.
這種想法肯定或遲或早會被社會所接受的。
It must have rained last night for it is so wet outside .
外面這么濕，昨晚肯定下雨了。
I didn't see you in class yesterday. You must have been
absent .我昨天上課沒見著你，你肯定沒有來。
Ought （后接帶to的不定式構成謂語動詞）應該
1）表示出于職責、義務該做某事
You ought not to be so careless in your work .你不應該在工作上如此粗心大意。
We ought to study hard for the sake of our motherland.
我們應該為了我們的祖國而努力學習。
His doctor said to him that he oughtn't to smoke so much .
他的大夫說他不應該抽這么多煙。
Oughtn't you to give us a chance to try ?
你難道不應該給我們一個嘗試的機會嗎？
2) 表示極有可能發生某事
As an auto repairman , Dick ought to know what is wrong with the car .
狄克作為一個汽車修理工總該知道這部車的毛病所在吧。
If we set off right now , we ought to be able to get there in time .
如果我們馬上出發，我們應當能夠按時到達那里。
Tomorrow ought to be a nice day for we have such a fine sunset today .
今天有這么好的晚霞，明天準是個晴天。
need（用于否定句和疑問句）需要
There are still one and a half hours to go , we needn't be insuch a hurry .
還有一個半小時，我們無需如此慌慌張張。
I hardly need say how much we missed you .不用說我們是多么想念你啊。
Need you go so soon ?你需要這么早就去嗎？
You need have no anxiety on my account .你不必為我而著急。
dare , dared（用于否定句、疑問句和疑問句）敢
Dare you stay here alone in the night ? 你敢在夜里呆在這兒嗎？
Nobody dared mention that matter. 沒有人敢提那件事。
How dared you say that to her ? 你怎么敢對她說那件事？
Even if you dare do it , I won't allow you to because it's too dangerous .
即使你敢做，我也會讓你做那件事，因為太危險了
shall 用作情態動詞時,適用于第二人稱和第三人稱。
主要表示下面幾層意思：
1）許諾
You shall hear everything directly you come .你一來就可以聽到一切了。
"Whatever you want you shall have , "said the Fairy .
仙女說："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 .
我不想對你女兒太嚴厲，她不會太受壓的。
I promise that you shall see her again before long .
我保證你不久就能再見到她。
2）命令
You shall come to my office immediately .你必須馬上來我的辦公室。
She shall not stay in my garden .她不可以呆在我的花園里。
He shall not come into my study . 不許他進我的書房。
You shall do as you are told .按告訴你的那么做。
3）決心（表示某事一定會發生）
That day shall come .那一天一定會到來。
It has been decided that he shall be given the job .
肯定給他那份工作的事已經定下來了。
This law shall come into effect on January 1st 2001
這個法律于2001年元月1日生效。
4）規定
Each competitor shall wear a number .每個參賽者要佩戴一個數字標。
The renter shall keep the house in a neat and orderly state .
租賃者要保持房屋整潔有序。
The supplier shall make shipment before October and the purchaser shall make payment within 15 days after
receipt of the goods .賣方十月以前裝運，買方在收到貨物后十五天內付款。
