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談談can與be able to 用法上的區別

語中, can 與be able to在用法上既有相同的一方面, 又有不同的地方,這是應該引起注意的。當它們表示能力的時候，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在以下幾方面:

1. can只有兩種時態, 即can 和could, 而be able to 有多種時態, 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．表示過去通過努力終於做成了某事要用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 發生變化，所以形式比can 多．可以說：I can swim. I am able to swim. 但是不能說：All the people could escape from the big fire in time. 只能說：All the people were able to escape from the fire in time.

3．can能表猜測，be able to不能。can的這種用法主要用於疑問句和否定句。

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 表示對現在完成或過去動作的猜測，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，又可以用疑問或否定形式。如果用could表示現在“可能”，比can語氣更加委婉。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,這句話表示的可能性更小, 因為它表示了兩種可能性, 既 “可能是”也可能“不是”，而 must be 只表示了一種猜測。

could have done 這個句型有它的特殊含意，表示“本來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.

可能有同學會理解為 “不能因為他為我們大家做了這些事情而過分表揚。”其實這樣理解是錯的, 因為這裏的cannot be不是表 “不能”，而是表“不可能是”的意思, 因此這句話的意思是“他為我們大家做了這些事, 應該受到大大的表揚。” 即 “再怎麼表揚也不過分”。

4．can 可以表“允許”, 與may可以互換, 此時不能用be able to 代替。如：

   “Can / May I sit here？”“Yes, please.”

由於may not 有兩個意思, 即 “不可能”和“不許可”, 因此使用時要特別注意。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. 在口語中，表達這兩種不同的含義是通過句重音來解決的。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.)

而在書面表達時不可能使用句重音的手段, 所以最好用can’t代替may not來表示 “不可能”：

He can’t go home this weekend.

5. can 是情態動詞，只能用作謂語成分，be able to是普通動詞短語，既可以作謂語，也可以作非謂語。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 後邊接動詞不定式表示一種實際情況時，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. 一般說來, be able to 後邊的動詞不定式沒有被動語態。如:

I’m sure he is able to correct his mistakes in grammar.

不說: *I’m sure his mistakes are able to be corrected by him.

但是美國近來也出現了Being able to be hurt by things這種結構, 美國專家解釋為English is becoming looser. (英語變得越來越不嚴謹了), 中學生不宜模仿。

下面做幾個練習:

1. –Take a seat, please. You must be very tired after a day’s work.

  --Thank you, but I ______ tired.

   A. can’t be   B. am not  C. mustn’t be   D. may not

2.       I hope you _______ catch up with your classmates in a few weeks.

   A. to be able to   B. can   C. be able to   D. able to

3.       “What do you think of the news in today’s newspaper?”“It _______ be true.”

   A. mustn’t   B. isn’t be able to   C. can   D. can’t

4.       Through generations of hardworking, our country ________ produce IC with its own intellectual property rights.

   A. is able to   B. can   C. has been able to   D. could

5.       Our teacher was ill yesterday, and so we ________ have a day off.

   A. could  It means that teacher did it on purpose so that the students could have a day off.  B. can     C. were able to    D. may

6.       He studied very hard last term, so he ______ get the first prize in the final examination.

   A. could    B. was able to    C. might     D. would

7.       You _____________too careful in your lessons.

   A. are never able to be / mean sth or sb try to stop you / 也不可用於猜測      B. can never be        C. can be       D. must

8.       He must have worked out the problem last night, __________?

   A. mustn’t he    B. hasn’t he      C. didn’t he      D. can’t he

9.       Was he able to pass the test for the driver’s license? No, he ___________.

   A. was not able 表刻意   B. couldn’t    表是否  C. wasn’t able to      D. wasn’t able to pass

10.   I __________ make a living in the USA because I’m not good in English.

   A. am unable to        B. am not able to         C. can        D. am impossible to

參考答案

1. B   2. B   3. D   4. C   5. C   6. B   7. B   8. C   9. B   10. A