情態動詞的語法特徵：1) 情態動詞不能表示正在發生或已經發生的事情，只表示期待或估計某事的發生;2) 情態動詞 除ought 和have 外，後面只能接不帶to 的不定式。3) 情態動詞沒有人稱，數的變化，即情態動詞第三人稱單數不加-s。4) 情態動詞沒有非謂語形式，即沒有不定式，分詞，等形式。
一、 比較can 和be able to
1)can could 表示能力;可能 (過去時用could),
只用於現在式和過去式(could)。be able to可以用於各種時態。
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. 位於助動詞後。
b. 情態動詞後。
c. 表示過去某時刻動作時。
d. 用於句首表示條件。
e. 表示成功地做了某事時，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不表示時態
1)提出委婉的請求，(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，疑問句中表示推測或懷疑。
He couldn't be a bad man.
他不大可能是壞人。
二、比較may和might
1) 表示允許或請求;表示沒有把握的推測;may 放在句首，表示祝願。
May God bless you!
He might be at home.
注意： might 表示推測時，不表示時態。只是可能性比may 小。
2) 成語： may/might as well，後面接不帶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. 此句意可從後半句推出。
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，我只得半夜裡把醫生請來。(客觀上需要做這件事)
He said that they must work hard. 他說他們必須努力工作。(主觀上要做這件事)
2) have to有人稱、數、時態的變化，而must只有一種形式。但must 可用於間接引語中表示過去的必要或義務。
He had to look after his sister yesterday.
3) 在否定結構中： 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
1) must用在肯定句中表示較有把握的推測，意為"一定"。
2) must表對現在的狀態或現在正發生的事情的推測時， must 後面通常接系動詞be 的原形或行為動詞的進行式。
You have worked hard all day.You must be tired. 你辛苦幹一整天，一定累了。(對現在情況的推測判斷)
He must be working in his office. 他一定在辦公室工作呢。
比較：
He must be staying there.
他現在肯定呆在那裡。
He must stay there.
他必須呆在那。
3) must 表示對已發生的事情的推測時，must 要接完成式。
I didn't hear the phone. I must have been asleep. 我剛才沒有聽到電話，我想必是睡著了。
4) must表示對過去某時正發生的事情的推測，must 後面要接不定式的完成進行式。
——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. 如果湯姆五點才離開這兒，他此時一定還未到家。
五、表示推測的用法
can, could, may, might, must 皆可表示推測，其用法如下：
1)情態動詞+動詞原形。
表示對現在或將來的情況的推測，此時動詞通常為系動詞。
I don't know where she is, she may be in Wuhan.
2)情態動詞+動詞現在進行時。
表示對現在或將來正在進行的情況進行推測。
At this moment, our teacher must be correcting our exam papers.
這時，我們老師想必在批改試卷。
3)情態動詞+動詞完成時。
表示對過去情況的推測。
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工作了。
The road is wet. It must have rained last night.
地是溼的，昨天晚上一定下雨了。
4)情態動詞+動詞的現在完成進行時。
表示對過去正在發生事情的推測。
Your mother must have been looking for you.
你媽媽一定一直在找你。
5)推測的否定形式，疑問形式用can't, couldn’t表示。
Mike can't have found his car, for he came to work by bus this morning.
邁克一定還沒有找回他的車，因為早上他是坐公共汽車來上班的。
注意：could, might表示推測時不表示時態，其推測的程度不如 can, may。
六、情態動詞+ have +過去分詞
1) may(might) have + done sth, can (could) have + done sth 表示過去，推測過去時間裡可能發生的事情。
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，對過去時間裡可能發生的事情的推測，語氣較強，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
本應該做某事，而事實上並沒有做。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.(事實上已扔了。)
ought to 在語氣上比should 要強。
4) needn't have done sth 本沒必要做某事
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.
七、should 和ought to
should 和ought to 都為"應該"的意思，可用於各種人稱。
——Ought he to go?
——Yes. I think he ought to.
表示要求，命令時，語氣由 should(應該)、had better最好)、must(必須)漸強。
八、 had better表示"最好"
had better 相當於一個助動詞，它只有一種形式，它後面要跟動詞原形。
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表示與事實相反的結果，意為"本來最好"。
You had better have come earlier.
九、 would rather表示"寧願"
would rather do
would rather not do
would rather… than… 寧願……而不願。
還有would sooner, had rather, had sooner都表示"寧願"、"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.
典型例題
---- Shall we go skating or stay at home?
----Which ___ do?
A. do you rather B. would you rather C. will you rather D. should you rather
答案B。本題考查情態動詞rather的用法，would rather +do sth 意為"寧願"，本題為疑問句， would 提前，所以選B。
十、 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…? 表示肯定含義的請求勸說時，疑問句中一般用some, 而不是any。
Would you like some cake?
3)否定結構中用will，一般不用would, won't you是一種委婉語氣。
Won't you sit down?
十一、 情態動詞的回答方式
問句 肯定回答 否定回答
Need you…? Yes, I must. No,I needn't
Must you…? /don't have to.
典型例題
1)——Could I borrow your dictionary?
——Yes, of course, you____.
A. might B. will C. can D. should
答案C.could表示委婉的語氣，並不為時態。答語中of course，表示肯定的語氣，允許某人做某事時，用can和 may來表達，不能用could或might。複習： will 與you連用，用來提出要求或下命令。should與you 連用，用來提出勸告。
2)——Shall I tell John about it?
——No, you ___. I've told him already.
A. needn't B. wouldn’t C. mustn't D. shouldn’t
答案A。needn't 不必，不用。 wouldn’t 將不, 不會的。 mustn't 禁止、不能。 shouldn’t 不應該。本題為不需要，不必的意思，應用needn't。
3)——Don't forget to come to my birthday party tomorrow.
——______.
A. I don't B. I won’t C. I can't D. I haven’t
答案B. will既可當作情態動詞，表請求、建議、也可作為實義動詞表"意願、意志、決心"，本題表示決心，選B。
十二、 帶to 的情態動詞
帶to 的情態動詞有四個：ought to, have to, used to, be to, 如加上have got to ,(=must), be able to，為六個。它們的疑問，否定形式應予以注意：
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 本身作為情態動詞使用。其他的詞作為實義動詞使用，變疑問，否定時，須有do 等助動詞協助。 典型例題
Tom ought not to ___ me your secret, but he meant no harm.
A.have told B.tell C.be telling D. having told
答案A。由於後句為過去時，告訴秘密的動作又發生在其前因，此地應用過去完成時，但它在情態動詞 ought to 後，所以用 have。
十三、比較need和dare
這兩詞既可做實義動詞用，又可做情態動詞用。作為情態動詞，兩者都只能用於疑問句，否定句和條件句。 need 作實義動詞時後面的不定式必須帶to，而dare作實義動詞用時, 後面的to 時常可以被省略。
1) 實義動詞： need (需要, 要求)
need + n. / to do sth
2) 情態動詞： need，只用原形need後加do，否定形式為need not。
Need you go yet?
Yes, I must. / No, I needn't.
3) need 的被動含義：need, want, require, worth(形容詞)後面接doing也可以表示被動：
need doing = need to be done