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be able to...能够(强调能力，able就是有能力的之意）。虽然情态助动词can同形容词短语be able to 均可用来表示“能够”之意,但在使用中应主要注意以下几点
1) 用 be able to表达将来做某事的能力,而不用can,如:
I'll be able to speak German in another few months.
再过几个月我就能说德语了。
can常用来表达现在的或一般的能力,如:
You can certainly cook, even if you cant do anything else.
即使其他事你都不会做,你也肯定会做饭。
但是,当含有现在对将来能力做出决定的意义时,可以用can
如
We are too busy today, but we can repair your car tomorrow.
我们今天太忙,不过我们明天能修你的车。
2) could 通常表示过去的一般能力： He could ruan faster than anyone else. 以下2种could的用法通常含有个人情感因素： could可以表示目前或将来的可能性(类似would）,如: 300, 000 jobs could be lost. 可能会有30万人失业。 There could be something wrong in the plan. 计划中可能有误。 could可以和see、hear和 smell这类词连用,表示通过感官了解某事,如 I could smell gas in the room. 我能闻到房间里有煤气的味道。 I could only see a few stars in the sky. 我在天空中只看到几颗星星。
3) 谈及过去特定场合能做某事时,通常使用was/ were able to, managed to或 succeeded in,而不用 could。
4) 可以用was/ were able to表达 could+完成时态结构表达的句义,即用来表达过去本能做某事而未做之意,如:
I was able to marry anybody I wanted to, but I didn't.(= I could have married anybody I wanted.)
我本能同我愿意的任何人结婚,但我没这样做
*
而应说
He tried to pursuade her but failed
他试图说服她,但未成功。
此外, manage指一次性行为,因此不可用其表示经常性行为,
如不可说
*He was a terrific liar,he managed to make anybody believe him.
而应说
He was a terrific liar,he could make anybody believe.
他是个了不得的骗子,他能让任何人都相信他。
相反,不可用can的过去式表示一次性行为,如不可说:
*
而应说
I talked for a long time, and in the end I managed to make her believe.
我说了许久,最终设法使她相信了我。
最后体会下面几句：
I couldn't swim very well, but I managed to swim across the small stream.
She couldn't speak English very well, but she managed to make herself understood.
I could swim very well when I was younger.
After trying to pick the lock for ages, she was finall able to open the door and escape.
My brother could solve difficult maths problems even when he was very young.
I finally managed to solve the problem.
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