談?wù)刢an與be able to 用法上的區(qū)別
語(yǔ)中,can 與be able to在用法上既有相同的一方面,又有不同的地方,這是應(yīng)該引起注意的.當(dāng)它們表示能力的時(shí)候,是同義的.如：
He can speak two foreign languages.
He is able to speak two foreign languages.
它們用法上的不同主要體現(xiàn)在以下幾方面:
1.can只有兩種時(shí)態(tài),即can 和could,而be able to 有多種時(shí)態(tài),如was/were able to,will/shall be able to,have/has been able to等:
I’ll be able to drive the car in a week.
Through his diligent work,the deaf-mute has been able to read and write in a sign language.
2．表示過(guò)去通過(guò)努力終于做成了某事要用be able to,而不能用can.如：
I was able to swim to the bank after the boat turned over.
can 和be able to 都可以表示能力.can泛指一般的能力,而且只有兩種形式,即:can,could．be able to 則主要指具體做到了某件事的能力,其形式主要是靠be 發(fā)生變化,所以形式比can 多．可以說(shuō)：I can swim.I am able to swim.但是不能說(shuō)：All the people could escape from the big fire in time.只能說(shuō)：All the people were able to escape from the fire in time.
3．can能表猜測(cè),be able to不能.can的這種用法主要用于疑問(wèn)句和否定句.
Somebody is knocking at the door.Who can it be?
It can’t be our teacher who is knocking at the door.
其肯定形式是must be.如:
It must be our teacher who is knocking at the door.
must have done something 表示對(duì)現(xiàn)在完成或過(guò)去動(dòng)作的猜測(cè),其否定形式是can’t have done.如:
“His parents must have gone abroad.” “No,I don’t think so.They can’t have gone,for I saw them only this morning.”
要特別注意它們的反意問(wèn)句形式：
He must be a good teacher,mustn’t /isn’t he?
The students must have gone to bed,haven’t they?
They must have finished their lessons yesterday,didn’t they?
could既可以用肯定形式,又可以用疑問(wèn)或否定形式.如果用could表示現(xiàn)在“可能”,比can語(yǔ)氣更加委婉.如：
If you don’t spend enough time on your studies,you could fail in the final examination.
It could be our teacher who turned off the light last night.
與It must be our teacher.相比,這句話(huà)表示的可能性更小,因?yàn)樗硎玖藘煞N可能性,既 “可能是”也可能“不是”,而 must be 只表示了一種猜測(cè).
could have done 這個(gè)句型有它的特殊含意,表示“本來(lái)可能做到而未做到的事”.如：
Our class team could have won the game.
再看以下例句：
He cannot be over praised for what he has done for all of us.
可能有同學(xué)會(huì)理解為 “不能因?yàn)樗麨槲覀兇蠹易隽诉@些事情而過(guò)分表?yè)P(yáng).”其實(shí)這樣理解是錯(cuò)的,因?yàn)檫@里的cannot be不是表 “不能”,而是表 “不可能是” 的意思,因此這句話(huà)的意思是“他為我們大家做了這些事,應(yīng)該受到大大的表?yè)P(yáng).” 即 “再怎么表?yè)P(yáng)也不過(guò)分”.
4．can 可以表 “允許”,與may可以互換,此時(shí)不能用be able to 代替.如：
“Can / May I sit here?” “Yes,please.”
由于may not 有兩個(gè)意思,即 “不可能” 和 “不許可”,因此使用時(shí)要特別注意.如：
He may not go home this weekend.
此句既可以表示 He will probably not go home this weekend.也可以表示 I don’t permit him to go home this weekend.在口語(yǔ)中,表達(dá)這兩種不同的含義是通過(guò)句重音來(lái)解決的.如：
He may `not go home this weekend.(I don’t permit him to go home this weekend.)
He `may not go home this weekend.(He will probably not go home this weekend.)
而在書(shū)面表達(dá)時(shí)不可能使用句重音的手段,所以最好用can’t代替may not來(lái)表示 “不可能”：
He can’t go home this weekend.
5.can 是情態(tài)動(dòng)詞,只能用作謂語(yǔ)成分,be able to是普通動(dòng)詞短語(yǔ),既可以作謂語(yǔ),也可以作非謂語(yǔ).如：
Being able to speak English fluently,he has no difficulty in communicating with the Americans at the party.
How I long to be able to communicate with the foreigners in fluent English!
6.be able to 后邊接動(dòng)詞不定式表示一種實(shí)際情況時(shí),其否定形式不是be not able to,而是cannot.如：
Were you able to catch the first bus yesterday morning?
No,I couldn’t.
7.一般說(shuō)來(lái),be able to 后邊的動(dòng)詞不定式?jīng)]有被動(dòng)語(yǔ)態(tài).如:
I’m sure he is able to correct his mistakes in grammar.
不說(shuō):*I’m sure his mistakes are able to be corrected by him.
但是美國(guó)近來(lái)也出現(xiàn)了Being able to be hurt by things這種結(jié)構(gòu),美國(guó)專(zhuān)家解釋為English is becoming looser.(英語(yǔ)變得越來(lái)越不嚴(yán)謹(jǐn)了),中學(xué)生不宜模仿.