I. 情态动词dare和need的用法
1. dare的用法
① dare 表示“敢”的意思，它的过去时是dared；
②除在”I dare say.”这种结构中，dare极少用作肯定结构中的情态动词；
③一般用在否定句和疑问句中，其用未能和其他情态动词一样，即dare不随人称变化而变，后面所跟的动词不定式不带to。如：
I dare not tell the truth. 我不敢照实说出。
Dare she go out alone at night?她敢一个人夜间出去吗？
Dare也有实义动词形式，在现在英语中，dare用作实义动词的时候更多一些。
2. need的用法
need的基本词义是“需要”，它既可作情态动词，又可作实义动词。
（1）need用作情态动词时，只用于否定句和疑问句。它只有一种形式，后接动词原形。
例如：
I don’t believe you needworry. 我相信你没有必要焦急。
—Need we go so soon? 我们需要这么早就去吗？
—Yes, we must. / No, we needn’t.是的，必须。／不，不必。
So I needn’t tell him, needI? 所以我不需要告诉他，对吧？
请比较以下两句话的不同意思：
You needn’t buy the coat. 你没有必要买那件外衣。（你还没买）
You needn’t have bought thecoat. 你没有必要买这件外衣的。（而你却买了）
【注意】
—Need I/we……?
—Yes, you must./—No, you needn’t/ don’t have to.
（2）need用作实义动词时，有时态、人称和数的变化。
如果人做主语，一般后接带to的动词不定式，即need to do sth.；
如果物做主语，一般后用need to be done或need doing sth.；
例如：
I need to look up this word inthe dictionary.
My watch needs mending/to be mended.我的手表需要修理。
We don’t need to work today.(=We needn’t work today.)
Does he need to go right now?(=Need he go right now?)
We need some more hands. 我们需要更多的帮手。
The garden needs cleaning.(=The garden needs to be cleaned.) 院子要清扫了。
The flowers need watering.(=The flowers need to be watered.) 花要浇水了。
II. 情态动词must, may, might, could, can表示推测：
以must为例，
must + do (be) 是推测现在存在的一般状态进行；
must + be doing 推测可能正在进行的事情；
must +have done 是推测可能已经发生过的事情。
1. must“肯定，一定”“”语气强，只用于肯定句中。
He must be a man from America.
He must be talking with hisfriend.
He must have already arrivedthere.
2. may和might“也许”，后者语气弱，更没有把握。可用于肯定句和否定句。
He may not be at home.
They might have finished theirtask.
3. can和could“可能”，could表示可疑的可能性，不及can’t语气强，用于肯定、否定、疑问句中。
The weather in that city couldbe cold now.
We could have walked there; itwas so near.（推测某事本来可能发生，但实际上没有发生）
Can he be in the office now? No,he can’t be there, for I saw him in the library just now. （语气很强，常用于疑问句和否定句中）
【注意】
1. 对现在的事实进行推测
主要结构：must/may/might+动词原形
be+名词/形容词/介词短语
be+doing
例句：
1. You must be Jeanne. I’m MathildeLoisel. We used to know each other very well.
2. They must be in bed alreadyat this time of the night.
3. The teacher must be joking.
4. Freda isn’t in class. She mustbe sick.
5. There must be something wrong.
6. She might be very clever, butshe hasn’t got much common sense.
7. He may be arriving this evening.
8. He may be traveling aroundthe world.
9. The keys can’t be in the room.I have just searched it very carefully.
10. Can the news be true?
2. 对过去的事实进行推测
结构：情态动词+have done / been+名词/形容词/介词短语。
例句：
1. Mrs. Longmans must have beena pretty girl in her youth.
2. He couldn’t have seen Annayesterday. She’s gone abroad.
3. I think I must have left myglasses in the library.
4. He might have overslept again.
5. Where can Tom have gone?
III.情态动词+have done用法
| 情态动词+ have done | 用法 | 例句 |
| must have done | 一定已经做了 | They must have finished their work. |
| can’t have done | 不可能已经做了 | They can’t have finished their work. |
| could have done | 可能已经做了或本来可能做但未做 | You could have used my computer because I didn’t use it. |
| may have done | 也许已经做了 | They may have finished their work. |
| might have done | 也许已经做了或本来可以做但未做 | You might have used my computer because I didn’t use it. |
| Should/ought to have done | 本来该做但未做 | You should have studied hard. |
| need not have done | 本来不必做但做了 | You need not have gone out. |
| would rather have done | 过去宁愿做但未做 | I would rather not have gone there. |
| would like to have done | 过去想做但未做 | I would like to have seen the film. |
| had better have done | 当时最好做了某事 | You’d better have gone there. |
