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204 can—could—be able to
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can和could都是情态动词，都和动词原形构成谓语。can的否定式为cannot（不能分开写），在口语中多用can’t这个形式，在英国读作/kɑːnt/，在美国读作/kænt/。could的否定式为could not，常可紧缩为couldn’t，读作/ˈkudnt/。
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在表示“能够”、“有能力做某事”时，can, could和be able to意思是差不多的。can说明现在情况：
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She can sing this song in French．她能用法语唱这支歌。
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（也可说：She is able to sing this song in French.）
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Can you translate this sentence into English？你能把这个句子译为英语吗？
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（也可说：Are you able to translate this sentence into English?）
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Sorry I can’t answer your question．对不起我不能回答你的问题。
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（也可说：Sorry I’m not able to answer your question.）
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could可用作can的过去式，说明过去的情况：
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She could swim when she was five．她5岁时就可以游泳了。
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（也可说：She was able to swim when she was five.）
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Could you run as fast as you do now？那时你能跑得像现在这样快吗？
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（也可说：Were you able to run as fast as you do now?）
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She couldn’t understand his state of mind．她不能理解他的心理状态。
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（也可说：She wasn’t able to understand his state of mind.）
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但be able to可以更明确地表示时间关系（可用于多种时态）：
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Will you be able to join us？你能参加我们的活动吗？
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We won’t be able to catch that train．我们赶不上那班火车。
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She said she wouldn’t be able to go with us．她说她无法和我们一道去。
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I haven’t been able to do that up till now．到现在为止我还没能这样做。
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She said she hadn’t been able to get in touch with them．她说她还未能和他们联系上。
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be able to还可和may, might, should等情态动词一起用：
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I may be able to get a ticket for you．我可能能给你弄到一张票。
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She might be able to help us．她有可能帮我们的忙。
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