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。
一、 比較can 和be able to
1）can could 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.
注意：could不表示時(shí)態(tài)
1）提出委婉的請(qǐng)求，（注意在回答中不可用could）。
—— Could I have the television on?
—— Yes, you can. / No, you can't.
2）在否定，疑問(wèn)句中表示推測(cè)或懷疑。
He couldn't be a bad man.
他不大可能是壞人。
二、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.
典型例題
Peter ___come with us tonight, but he isn't very sure yet.
A. must B. may C. can D. will
答案B. 表可能性只能用may. 此句意可從后半句推出。
三、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。
四、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。
五、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。
六、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 I needn'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.
一、 比較can 和be able to
1）can could 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.
注意：could不表示時(shí)態(tài)
1）提出委婉的請(qǐng)求，（注意在回答中不可用could）。
—— Could I have the television on?
—— Yes, you can. / No, you can't.
2）在否定，疑問(wèn)句中表示推測(cè)或懷疑。
He couldn't be a bad man.
他不大可能是壞人。
二、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.
典型例題
Peter ___come with us tonight, but he isn't very sure yet.
A. must B. may C. can D. will
答案B. 表可能性只能用may. 此句意可從后半句推出。
三、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。
四、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。
五、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。
六、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 I needn'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.