1.情態(tài)動(dòng)詞的語(yǔ)法特征
1) 情態(tài)動(dòng)詞不能表示正在發(fā)生或已經(jīng)發(fā)生的事情，只表示期待或估計(jì)某事的發(fā)生。
2) 情態(tài)動(dòng)詞 除ought 和have 外，后面只能接不帶to 的不定式。
3) 情態(tài)動(dòng)詞沒(méi)有人稱(chēng)，數(shù)的變化，即情態(tài)動(dòng)詞第三人稱(chēng)單數(shù)不加-s。
4) 情態(tài)動(dòng)詞沒(méi)有非謂語(yǔ)形式，即沒(méi)有不定式，分詞，等形式。
2.比較can 和be able to
1)cancould 表示能力;可能 (過(guò)去時(shí)用could),
只用于現(xiàn)在式和過(guò)去式(could)。be able to可以用于各種時(shí)態(tài)。
They will be able to tell you the news soon. 他很快就能告訴你消息了。
2)只用be able to
a. 位于助動(dòng)詞后。b. 情態(tài)動(dòng)詞后。c. 表示過(guò)去某時(shí)刻動(dòng)作時(shí)。
d. 用于句首表示條件。e. 表示成功地做了某事時(shí)，只能用was/were able to，不能用could。
He was able to flee Europe before the war broke out.
= He managed to flee Europe before the war broke out.
3.比較may和might
1) 表示允許或請(qǐng)求;表示沒(méi)有把握的推測(cè);may 放在句首，表示祝愿。
May God bless you! He might be at home.
注意： might 表示推測(cè)時(shí)，不表示時(shí)態(tài)。只是可能性比may 小。
2) 成語(yǔ)： may/might as well，后面接不帶to 的不定式，意為"不妨"。
If that is the case, we may as well try.
4.比較have to和must
1)兩詞都是’必須’的意思，have to 表示客觀的需要， must 表示說(shuō)話人主觀上的看法，既主觀上的必要。
My brother was very ill, so I had to call the doctor in the middle of the night.我弟弟病得很厲害，我只得半夜里把醫(yī)生請(qǐng)來(lái)。(客觀上需要做這件事)
He said that they must work hard. 他說(shuō)他們必須努力工作。(主觀上要做這件事)
2)have to有人稱(chēng)、數(shù)、時(shí)態(tài)的變化，而must只有一種形式。但must 可用于間接引語(yǔ)中表示過(guò)去的必要或義務(wù)。
He had to look after his sister yesterday.
3) 在否定結(jié)構(gòu)中： don’t have to 表示"不必" mustn’t 表示"禁止"
You don’t have to tell him about it. 你不一定要把此事告訴他。
You mustn’t tell him about it.你一定不要把這件事告訴他。
5.must表示推測(cè)
1) must用在肯定句中表示較有把握的推測(cè)，意為"一定"。
2) must表對(duì)現(xiàn)在的狀態(tài)或現(xiàn)在正發(fā)生的事情的推測(cè)時(shí)， must 后面通常接系動(dòng)詞be 的原形或行為動(dòng)詞的進(jìn)行式。
You have worked hard all day. You must be tired.
你辛苦干一整天，一定累了。(對(duì)現(xiàn)在情況的推測(cè)判斷)
He must be working in his office. 他一定在辦公室工作呢。
比較：
He must be staying there. 他現(xiàn)在肯定呆在那里。
He must stay there. 他必須呆在那。
3) must表示對(duì)已發(fā)生的事情的推測(cè)時(shí)，must 要接完成式。
I didn’t hear the phone. I must have been asleep.
我剛才沒(méi)有聽(tīng)到電話，我想必是睡著了。
4) must表示對(duì)過(guò)去某時(shí)正發(fā)生的事情的推測(cè)，must 后面要接不定式的完成進(jìn)行式。
---Why didn’t you answer my phone call?
---Well, I must have been sleeping, so I didn’t hear it.
5) 否定推測(cè)用can’t。
If Tom didn’t leave here until five o’clock, he can’t be home yet.
如果湯姆五點(diǎn)才離開(kāi)這兒，他此時(shí)一定還未到家。
6.表示推測(cè)的用法
can, could, may, might, must 皆可表示推測(cè)，其用法如下：
1)情態(tài)動(dòng)詞+動(dòng)詞原形。
表示對(duì)現(xiàn)在或?qū)?lái)的情況的推測(cè)，此時(shí)動(dòng)詞通常為系動(dòng)詞。
I don’t know where she is, she may be in Wuhan.
2)情態(tài)動(dòng)詞+動(dòng)詞現(xiàn)在進(jìn)行時(shí)。
表示對(duì)現(xiàn)在或?qū)?lái)正在進(jìn)行的情況進(jìn)行推測(cè)。
At this moment, our teacher must be correcting our exam papers.
這時(shí)，我們老師想必在批改試卷。
3)情態(tài)動(dòng)詞+動(dòng)詞完成時(shí)。
表示對(duì)過(guò)去情況的推測(cè)。
We would have finished this work by the end of next December.
明年十二月底前我們很可能已完成這項(xiàng)工作了。
The road is wet. It must have rained last night. 地是濕的，昨天晚上一定下雨了。
4)情態(tài)動(dòng)詞+動(dòng)詞的現(xiàn)在完成進(jìn)行時(shí)。
表示對(duì)過(guò)去正在發(fā)生事情的推測(cè)。
Your mother must have been looking for you. 你媽媽一定一直在找你。
5)推測(cè)的否定形式，疑問(wèn)形式用can’t, couldn’t表示。
Mike can’t have found his car, for he came to work by bus this morning.
邁克一定還沒(méi)有找回他的車(chē)，因?yàn)樵缟纤亲财?chē)來(lái)上班的。
注意：could, might表示推測(cè)時(shí)不表示時(shí)態(tài)，其推測(cè)的程度不如can, may。
7.情態(tài)動(dòng)詞+ have +過(guò)去分詞
1) may(might) have + done sth, can (could) have + done sth表示過(guò)去，推測(cè)過(guò)去時(shí)間里可能發(fā)生的事情。
Philip may (might) have been hurt seriously in the car accident.
Philip can (could) have been hurt seriously in the car accident.
2)must have +done sth，對(duì)過(guò)去時(shí)間里可能發(fā)生的事情的推測(cè)，語(yǔ)氣較強(qiáng)，具有"肯定"，"諒必"的意思。
Linda has gone to work, but her bicycle is still here.
She must have gone by bus.
3) ought to have done sth, should have done sth
本應(yīng)該做某事，而事實(shí)上并沒(méi)有做。否定句表示"不該做某事而做了"。
You ought to (should) have been more careful in this experiment.
He ought not to have thrown the old clothes away.(事實(shí)上已扔了。)
ought to 在語(yǔ)氣上比should 要強(qiáng)。
4) needn’t have done sth 本沒(méi)必要做某事
I dressed very warmly for the trip, but Ineedn’t have done so. The weather was hot.
5) would like to have done sth 本打算做某事
I would like to have read the article, but I was very busy then.
8.should 和ought to
should 和ought to 都為"應(yīng)該"的意思，可用于各種人稱(chēng)。
---Ought he to go?
---Yes. I think he ought to.
表示要求，命令時(shí)，語(yǔ)氣由 should(應(yīng)該)、had better最好)、must(必須)漸強(qiáng)。
9.had better表示"最好"
had better 相當(dāng)于一個(gè)助動(dòng)詞，它只有一種形式，它后面要跟動(dòng)詞原形。
had better do sth
had better not do sth
It is pretty cold. You’d better put on my coat.
She’d better not play with the dog.
had better have done sth表示與事實(shí)相反的結(jié)果，意為"本來(lái)最好"。
You had better have come earlier.
10.would rather表示"寧愿"
would rather do
would rather not do
would rather… than… 寧愿……而不愿。
還有would sooner, had rather, had sooner都表示"寧愿"、"寧可"的意思。
If I have a choice, I had sooner not continue my studies at this school.
I would rather stay here than go home. = I would stay here rather than go home.
11.will和would
注意：
1)would like; Would like to do = want to 想要，為固定搭配。
Would you like to go with me?
2)Will you…?Would you like…? 表示肯定含義的請(qǐng)求勸說(shuō)時(shí)，疑問(wèn)句中一般用some, 而不是any。
Would you like some cake?
3)否定結(jié)構(gòu)中用will，一般不用would, won’t you是一種委婉語(yǔ)氣。
Won’t you sit down?
12.情態(tài)動(dòng)詞的回答方式
問(wèn)句肯定回答否定回答
Need you…?Yes, I must.No, I needn’t
Must you…?/don’t have to.
13.帶to 的情態(tài)動(dòng)詞
帶to 的情態(tài)動(dòng)詞有四個(gè)：ought to, have to, used to, be to, 如加上have got to ,(=must), be able to，為六個(gè)。它們的疑問(wèn)，否定形式應(yīng)予以注意：
Do they have to pay their bill before the end of the month?
She didn’t use to play tennis before she was fourteen.
You ought not to have told her all about it.
Ought he to see a heart specialist at once?
ought to 本身作為情態(tài)動(dòng)詞使用。其他的詞作為實(shí)義動(dòng)詞使用，變疑問(wèn)，否定時(shí)，須有do 等助動(dòng)詞協(xié)助。
14.比較need和dare
這兩詞既可做實(shí)義動(dòng)詞用，又可做情態(tài)動(dòng)詞用。作為情態(tài)動(dòng)詞，兩者都只能用于疑問(wèn)句，否定句和條件句。 need 作實(shí)義動(dòng)詞時(shí)后面的不定式必須帶to，而dare作實(shí)義動(dòng)詞用時(shí), 后面的to 時(shí)常可以被省略。
1) 實(shí)義動(dòng)詞： need(需要, 要求)
need + n. / to do sth
2) 情態(tài)動(dòng)詞： need，只用原形need后加do，否定形式為need not。
Need you go yet?
Yes, I must. / No, I needn’t.
3) need 的被動(dòng)含義：need, want, require, worth(形容詞)后面接doing也可以表示被動(dòng)：
need doing = need to be done.
