2.can与be able to用法辨析

(1)在许多情况下可以交替使用。

As I have got enough money，I can/am able to help her．

我有足够的钱，因此我能够帮助他。

If you don't work hard，you can't/won't be able to pass the test．

你要是不努力，就通过不了考试。

Will you be able to get up at six?

=Can you get up at six?

你能六点钟起床吗?

(2)be able to有更多的时态形式，也可以有非谓语形式。

No one in my family has been able to get a job since dad lost hismonths ago．

自从我父亲一个月前失业以来，我家里没有人能够找到工作。

You won't be able to catch him．

你是赶不上他的。

He told me that he had been able to get along well with her．

他告诉我说他跟她关系一直不错。

(3)be able to之后的动词不用被动语态。例如不可以说:

This work is able to be done by hand．

She is able to be told by telephone．

(4)be able to的主语只能是有生命的词，例如不可以说:

The engine is able to work．

(5)在肯定句或疑问句中，表示具有某种知识或技能，固而能够完成一件事情(意义是具有这种能力，而不是已经做成了这件事)时，用could或was(were)able to均可;表示已经做成了一件事，即表示一种成就，相当于managed to或succeeded in(意义是“得以”)时，那就只能用was(were)able to，不能用could。即是说，was(were)able to可以当managed to或succeeded in讲，而could不能。

The treatment was successful and I was able to go home after exactly fourteen months，two days and six hours in hospital．(人教大纲版高二上第55页)

治疗成功了，我在十四个月零两天六小时之后出院回家了。

×The treatment was successful and I could go home after exactly fourteen months，two days and six hours in hospital．

He was able to save the drowning boy and so he was thought highly of．

他救起了那个溺水的男童，得到了大家的好评。

×He could save the drowning boy and so he was thought highly of．

提示：

但在否定句中，could和was(were)able to都具有这一意义，couldn't可以与wasn't/weren't able to互换。

He couldn't/wasn't able to lift the iron so often．

他并不能一再举起那块铁。

在含有否定意义的句子(不是否定句)里，could也可能具有这一意义。How long did it take before you could answer a question like that?

你花了多长时间才回答出这样一个问题?

此句含有“不花那么多时间就回答不出”的意思，could显然是表示成就。又:

We asked everyone there if they had found a necklace，but without luck．We couldn't find it．(人教大纲版高一下第18页)

=We asked everyone there if they had found a necklace，but without luck．We weren't able to find it．

我们问遍了那里所有的人，问他们有没有拾到一条项链，但运气不好。我们没能找到。