英語語法培訓之情態動詞用法
英語的朋友們應該知道情態動詞吧? 情態動詞(Modal verbs)本身有一定的詞義，表示語氣的單詞。但是不能獨立作謂語，只能和動詞原形一起構成謂語。情態動詞用在行爲動詞前，表示說話人對這一動作或狀態的看法或主觀設想。今天來看看他們的用法吧。
情態動詞雖然數量不多，但用途廣泛，主要有下列：can (could), may (might), must, need, ought to, dare (dared), shall (should), will (would)。
因爲情態動詞帶有某種的情緒性，比如dare就很強硬，比較負面，而may就比較的和善，爲了避免在談話時把“討論”變成“攻擊”。
分類
情態動詞有四類：
①只做情態動詞：
must,can(could),may(might),ought to
②可做情態動詞又可做實義動詞：
need,dare
③可做情態動詞又可做助動詞：
shall(should),will(would)
④具有情態動詞特徵：
have(had) to,used to
位置
情態動詞在句中放在謂語動詞之前, 謂語動詞前若有助動詞,則在助動詞之前,疑問句中，情態動詞則在主語之前。
I can see you. Come here.
我能看見你，過來吧。
He must have been away.
他一定走了。
What can I do for you?
我能幫你嗎?
How dare you treat us like that!
你怎能那樣對待我們!
特點
情態動詞無人稱和數的變化, 情態動詞後面跟的動詞需用原形,否定式構成是在情態動詞後面加 "not"。
個別情態動詞有現在式和過去式兩種形式, 過去式用來表達更加客氣, 委婉的語氣, 時態性不強, 可用於過去，現在或將來。情態動詞屬非及物動詞，故沒有被動語態。情態動詞沒有非謂語形式，即沒有不定式，分詞，等形式。
He could be here soon.
他很快就來。
We can t carry the heavy box.
我們搬不動那箱子。
I m sorry I can t help you.
對不起，我幫不上你。
基本助動詞與情態助動詞最主要的區別之一是，基本助動詞本身沒有詞義，而情態助動詞則有自己的詞義，能表示說話人對有關動作或狀態的看法，或表示主觀設想：
What have you been doing since? (構成完成進行體，本身無詞義)
I am afraid I must be going. (一定要)
You may have read some account of the matter. (或許已經)
除此之外，情態助動詞還有如下詞法和句法特徵：
1) 除ought和used以外，其他情態動詞後面只能接不帶to的不定式。如果我們把ought to和used to看做是固定詞組的話，那麼，所有情態動詞無一例外地只能接不帶to的不定式：
We used to grow beautiful roses.
I asked if he would come and repair my television set.
2) 情態助動詞在限定動詞詞組總是位居第一：
They need not have been punished so severely.
3) 情態助動詞用於第三人稱單數現在時的時候，沒有詞形變化，即其詞尾無-s形式：
She dare not say what she thinks.
4) 情態動詞沒有非限定形式，即沒有不定式和分詞形式，也沒有相應的動名詞：
Still, she needn t have run away.
5) 情態助動詞的“時”的形式並不是時間區別的主要標誌。在不少場合，情態助動詞的現在時和過去時形式都可以表示現在、過去或將來時間：
Would you mind very much if I ask you to do something?
She told him he ought not to have done it.
6) 情態助動詞之間是相互排斥的，即在一個限定動詞詞組中只能出現一個情態助動詞，但有時卻可以與have和be基本助動詞連用：
You should have washed the wound.
Well, you shouldn t be reading a novel.
用法
首先它是動詞，而且不同於行爲動詞，行爲動詞表示的是可以通過行爲來表達的動作(如寫，讀，跑)，而情態動詞只是表達的一種想法(如能，也許，敢)。
用法是：情態動詞+行爲動詞原形
例句：I can read this sentence in English.
我能用英語讀這句話。
情態動詞是一種本身有一定的詞義,表示說話人的情緒,態度或語氣的動詞,但不能單獨作謂語,只能和其他動詞原形構成謂語。
We can be there on time tomorrow.
我們明天能按時去那兒。
May I have your name?
我能知道你的名字嗎?
Shall we begin now?
我們現在就開始嗎?
You must obey the school rules.
你必須遵守校規。
功能
助動詞(auxiliary)主要有兩類：基本助動詞(primary auxiliary)和情態助動詞(modal auxiliary)。
基本助動詞有三個：do, have和be;
情態助動詞有十三個：may, might; can, could; will, would; shall, should; must, need, dare, used to, ought to.
上述兩類助動詞的共同特徵是，在協助主動詞構成限定動詞詞組時，具有作用詞的功能：
1) 構成否定式：
He didn t go and neither did she.
The meeting might not start until 5 o clock.
2) 構成疑問式或附加疑問式：
Must you leave right now?
You have been learning French for 5 years, haven t you?
3) 構成修辭倒裝：
Nowhere can he obtain any information about his sister.
Hardly had he arrived when she started complaining.
4) 代替限定動詞詞組：
A: Who can solve this crossword puzzle?
B: Tom can.
A: Shall I write to him?
B: Yes, do.
用法要點
can, could
1) 表示能力(體力、知識、技能)。
Can you lift this heavy box?(體力)
Mary can speak three languages.(知識)
Can you skate?(技能)
此時可用be able to代替。Can只有一般現在時和一般過去式;而be able to則有更多的時態。
I’ll not be able to come this afternoon.
當表示“經過努力才得以做成功某事”時應用be able to，不能用Can。
如：He was able to go to the party yesterday evening in spite of the heavy rain.
2) 表示請求和允許。
Can I go now?
Yes, you can. / No, you can’t.
此時可與may互換。在疑問句中還可用could,might代替，不是過去式，只是語氣更委婉，不能用於肯定句和答語中。
Could I come to see you tomorrow?
Yes, you can. ( No, I’m afraid not. )
3) 表示客觀可能性(客觀原因形成的能力)。
They’ve changed the timetable, so we can go by bus instead.
This hall can hold 500 people at least.
4) 表示推測(驚訝、懷疑、不相信的態度)，用於疑問句、否定句和感嘆句中。
Can this be true?
This can’t be done by him.
How can this be true?
特別說明：
(1) could用來表示請求時，語氣委婉，主要用於疑問句，不能用於肯定句，答語應用can(即：could不能用於現在時態的簡略答語中)。
Could I use your dictionary?
Yes, you can.(否定回答可用：No, I’m afraid not.)
此時可與may互換。在疑問句中還可用could,might代替，不是過去式，只是語氣更委婉，不能用於肯定句和答語中。
Could I come to see you tomorrow?
Yes, you can. ( No, I’m afraid not. )
(2) can和be able to辨析
can(could)和be able to都可以表示能力，意思上沒有區別。但can只有現在式和過去式，而be able to則有更多的形式。
如：
I’ve always wanted to able to speak fluent English.
Those bags look really heavy, are you sure you’ll be able to carry them on your own?
但是，表示在過去某時的某一場合經過一番努力，終於做成了某事，通常不用could，而用was/were able to來表示。could：有潛能，但並未做到，這時was/were able to相當於managed to do或succeed in doing。
如：
After the accident it was a long time before she was able to walk again.
The fire was very big, but most people were able to escape from the building.
I can sing many English songs.
我會唱許多英文歌曲。
We were able to return to our campsite before the heavy rain.
我們在下大雨前設法會到了野營地。
He was able to swim across the river and escaped being caught.
他游到了河對岸，沒有被抓住。
在否定句中，can/could與be able t幾乎沒有什麼差別，兩者可以互換。例如：
She wasn’t able to/couldn’t cook French dishes.
她不會做法國菜。
(3) 慣用形式“can not …too…”表示“無論怎麼……也不(過分)”。
如：
You cannot be too careful.
你越小心越好。
慣用形式“can not but+ 不定式(不帶to)”表示“不得不，只好”。如：
I can not but admire her determination.
我不得不欽佩你的決心。
may, might
1) 表示請求和允許。might比 may語氣更委婉，而不是過去式。否定回答時可用can’t或mustn’t，表示“不可以，禁止”。
Might/ May I smoke in this room?
No, you mustn’t.
May/Might I take this book out of the room?
Yes, you can. (No, you can’t / mustn’t. )
用May I...?徵徇對方許可時比較正式和客氣，而用Can I...?在口語中更常見。
2)用於祈使句，表示祝願。
May you succeed!
3) 表示推測、可能性(不用於疑問句)。
might不是過去式，它所表示的可能性比may小。
1.He may /might be very busy now.
2.Your mother may /might not know the truth.
must, have to
1) 表示必須、必要。
You must come in time.
在回答引出的問句時，如果是否定的，不能用mustn t(禁止，不準)，而用needn’t, don’t have to(不必).
Must we hand in our exercise books today?
Yes, you must.
No, you don’t have to / you needn’t.
2) must是說話人的主觀看法， 而have to則強調客觀需要。Must只有一般現在時， have to 有更多的時態形式。
1. He play isn’t interesting, I really must go now.
2. I had to work when I was your age.
3) 表示推測、可能性(只用於肯定的陳述句)
1. You’re Tom’s good friend, so you must know what he
likes best.
2. Your mother must be waiting for you now.
dare, need
1) dare作情態動詞用時, 常用於疑問句、否定句和條件從句中, 過去式形式爲dared。
1. How dare you say I’m unfair?
2. He daren’t speak English before such a crowd, dare he?
3. If we dared not go there that day， we couldn’t get the beautiful flowers.
2) need 作情態動詞用時, 常用於疑問句、否定句。在肯定句中一般用must, have to, ought to, should代替。
1.You needn’t come so early.
2. Need I finish the work today?
Yes, you must. / No, you needn’t.
3) dare和 need作實義動詞用時， 有人稱、時態和數的變化。在肯定句中，dare後面常接帶to的不定式。在疑問句和否定句中，dare後 面可接帶to或不帶to的不定式。而need後面只能接帶to的不定式。
1. I dare to swim across this river.
2. He doesn’t dare (to) answer.
3. He needs to finish his homework today.
shall, should
1) shall 用於第一人稱，徵求對方的意見。
What shall we do this evening?
2) shall 用於第二、三人稱，表示說話人給對方的命令、警告、允諾或威脅。
1. You shall fail if you don’t work hard.(警告)
2. He shall have the book when I finish it.(允諾)
3. He shall be punished.(威脅)
will, would
1) 表示請求、建議等，would更委婉。
Will / Would you pass me the ball, please?
2) 表示意志、願望和決心。
1. I will never do that again.
2. They asked him if he would go abroad.
3) would表示過去反覆發生的動作或某種傾向。would表示過去習慣時比used to正式，且沒有“現已無此習慣”的含義。
1. During the vacation, he would visit me every other day.
2. The wound would not heal.
4) 表示估計和猜想。
It would be about ten o’clock when she left home.
特別說明：would與used to辨析
would可用來表示過去反覆出現的動作，但不能表示過去存在的狀態，所以我們不能說：“she would be a quiet girl.”
另外，would強調過去某種特定情況下的活動，是完全過去的事情，同現在沒有聯繫。而used to則着眼於過去和現在的對比，隱含現在已不存在，動作或狀態都可表示。Would可以表示不規則的習慣，used to則不可。
如：
He used to be a naughty boy and cause trouble.
I used to get up at six in the morning.
Sometimes she would take a walk in the neighboring woods.
In those days, whenever I had difficulties, I would go to Mr. Chen for help.
should, ought to
1) should, ought to表示“應該”，ought to表示義務或責任，比should語氣重。
1. I should help her because she is in trouble.
2. You ought to take care of the baby.
2) 表示勸告、建議和命令。should, ought to可通用，但在疑問句中常用should。
1. You should / ought to go to class right away.
2. Should I open the window?
3) 表示推測，should , ought to (客觀推測), must(主觀推測)。
1.He must be home by now. (斷定他已到家)
2.He ought to/should be home by now.(不太肯定)
3. This is where the oil must be.(直爽)
4. This is where the oil ought to/should be.(含蓄)
情態動詞+不定式完成式(have done)
1) can / could + have done在肯定句中表示“本來可以做而實際上能做某事”，是虛擬語氣;在疑問句或否定句中表示對過去行爲的懷 疑或不肯定, 表示推測。
1. You could have done better, but you didn’t try your best.(虛擬語氣)
2. He can’t have been to that town.(推測)
3. Can he have got the book?(推測)
2) may / might +不定式完成式(have done)表示對過去行爲的推測。不能用於疑問句中，沒有虛擬語氣的用法。Might所表示的可能性比may小。
1. He may not have finished the work .
2. If we had taken the other road, we might have arrived earlier.
3)must +不定式完成式(have done)用於肯定句中，表示對過去行爲的推測。意爲“一定、想必”。其疑問、否定形式用can，can’t代替。參看1) can / could + have done表示推測。
1. You must have seen the film Titanic.
2. He must have been to Shanghai.
4)should +不定式完成式(have done)用於肯定句中，表示對過去行爲的推測。
He should have finished the work by now。
表示“本應該做而實際上沒有做某事”，其否定式表示某種行爲本不該發生卻發生了。可以與ought to +不定式完成式(have don e)互換。
1. You ought to / should have helped him. (but you didn’t.)
2. She shouldn’t have taken away my measuring tape, for I wanted to use it.
5) needn’t +不定式完成式(have done)表示“本來不必做而實際上做了某事”。
You needn’t have watered the flowers, for it is going to rain.
6) will +不定式完成式(have done)主要用於第二、三人稱，表示對已完成的動作或事態的推測。
He will have arrived by now.
情態動詞表示猜測
肯定：must>should>could>may>might
否定：can’t>shouldn’t>couldn’t>may not>
表示要求，命令時，語氣由 should(應該)、had better最好)、must(必須)漸強。
必須掌握情態動詞表示推測語氣時的反意疑問句。
例如：
The road is wet. It must have rained, hasn t it?
The road is wet. It must have rained last night, didn t it? (多了過去時的時間狀語)
I met him on his way home. He can’t be in the library now, isn t he?
The coat is too close to the fire. It might get burnt, doesn t it?
大家都看明白了嗎?是不是對自己平時的學習有了新的認識?如果你覺得自己的英語水平還不夠的話，一定要多多努力，上面的內容只是英語中的一個小小的知識點。大家要從這個知識點入手，學習更多更好更優質內容。
