否定must 表示“禁止，不允许，一定不要”
否定don't have to 表示“不必,不一定非得,不一定要”
We mustn't put off the trip no matter what happens.
We don't have to put off the trip, although it rains.
11. Must & have to时态的区别
Must只用于现在, 而have to则可用于不同时态。用had to表示过去 , 用will have to表示将来。
We had to resort to force in that case.
We will have to reconsider the whole thing if this plan fails.
One must have faith in life.
12. have to
have to的疑问句和否定句都要用助动词do/does，过去式则用did。
Do we have to wait for him to start dinner?
Does he have to show you the tickets to enter the cinema?
Did I have to do the other task as well?
You don't have to come in person.
She doesn't have to bring her dictionary.
We didn't have to make preparations for the quiz.
13. Need & dare
Need & dare 既能作实义动词，又能作情态动词。
Need & dare在肯定句中，通常为实义动词，后面的不定式要带to，单数第三人称加s，有时态变化。
Need & dare作情态动词时，多用于否定句和疑问句。
Examples
She needs to work even harder if she wants to succeed in her study.
He dares to speak the truth.
Do you need to take a break?
They didn't dare to disobey.
Need he leave so soon?
You needn't have done all the heavy work.
She dare not go home late at night.
How dare you say I am unfair?
14. might as well
might as well译为“不妨”表示规劝。
You might as well wait to see what I can do for you.
You might as well leave now.
We might as well try a second time to see what's wrong with the equipment.
15. be to 可视为情态动词
be to的三种意义
①系表结构
My duty is instruct students on what is wrong and what is right.
②将来时的特殊表达法
I was to go back home when the phone called.
③“应当”“规定是”可视为情态动词–be supposed to
You are to obey the school rules and regulations.
16. Would rather
表意愿：I would rather/sooner not go there on my own.
表选择：She would rather die than surrender to the enemy.
(句型Would rather do sth. than do sth. else)
表虚拟：would rather sb. did /didn't do sth.
I would rather you didn't do anything for the time being.
17. Used to & would
情态动词used to表“过去常常做”。往往含有“不再如此”之意。
Would只能表示动作，不能表示状态。而used to都可以。.
He would ask the teacher some strange questions.
He used to ask the teacher ……
He used to be much better off than he is now.
He would be much …… (wrong)
18. More Information
Would侧重叙述过去, 而used to 强调过去和现在的对比。
Sometimes she would give me some pleasant surprises.
My mother used to be hot-tempered, but now she is very mild.
情态动词的完成式
1. needn't have done
(1)表示过去本不必做的事情
You needn't have done all the calculation by yourself. We have computers to do that sort of work.
She needn't have risen so early. There was no hurry.
They needn't have brought so much food with them. We had enough for all the guests.
Must have done
(2) 对过去事情的肯定猜测
He must have worked very hard, for he did a good job in the last exam.
They must have had a quarrel. They hadn't talked with each other for nearly a week.
There must have been a blackout. The whole street was in darkness.
can't/couldn't have done
(3) 对过去事情的否定猜测
(must have done的否定形式)
It can't have been her. She has gone to Shanghai.
You can't have seen me in the meeting last week. I was on a business trip then.
He can't have worked hard. I have seen him playing with his neighbors very often.
could & may/might have done
(4) 表示对过去事情的可能性的推测
You may / might have been injured in the car accident.
You may have been hurt by the wild animal.
You may have read the book.
You might have been more careful.
2. will的完成式
(1)Will have done 表示将来完成时
By the end of this week, the building will have been completed.
By May or June, we will have already known whether China succeeds in bidding for the 2008 Olympic Games.
By next month, we will have learned the first 5 chapters.
(2)Would/could have done 表示对过去虚拟
You could/would have done much better if you had followed the teacher's instructions.
The earth could have been a better place if we had all known the importance of protecting the environment.
You couldn't have been so miserable had you listened to my advice.
She wouldn't have failed the English exam if she had done enough revision exercises.
3. should的完成式
should (ought to) have done 过去本该做某事
shouldn't (ought not to) have done 过去本不该做某事
You shouldn't have behaved so impolitely in front of all the guests.
We shouldn't have relied too much on him.
He shouldn't have cheated on the exam.
She shouldn't have agreed to marry such an irresponsible man.