英语中表示能力的情态动词■表示能力一般既可用can, 也可用be able to。如：Billy is only 9 months and he can already (或is already able to) stand up. 比利只有9个月，他就已经能够站起来了。Jim could (或was able to) run very fast when he was a boy. 吉姆小时候就能跑得很快。■但使用时仍需注意以下6点：1) can只有can, could两种形式，若需要将来式或完成式等其它时态形式时，只能用be able to。如：I have not been able to go to work for three days. 我三天没能去上班了。2) 表示在过去成功在完成某一具体动作时，通常不用could，可用was / were able to do, managed to do, succeed in doing等。The fire spread through the hotel very quickly but everyone ________ get out. (全国卷) A. had to B. would C. could D. was able to【分析】答案选D。这是指在具体的某次火灾中人们能够逃出去。3) be able to可以与其它情态动词连用，但can不能。如：He ought to be able to pass the exam this time. 这次他应当能够通过考试。4) 表示说话时正在发生的事时，一般不用be able to。如Look! I can stand on my hands. 看！我能倒立。5) 当无生命的事物作主语时，可用can，但不用be able to。如：A computer ________ think for itself; it must be told what to do. . (全国卷)A. can’t B. couldn’t C. may not D. might not【分析】答案选A. 由后文可知电脑没有独立思考的能力，又是一般现在时，所以用can表示能力。主语是无生命的事物。6) 被动结构中可用can，但不能用be able to。如：Coal can be used to produce electricity for agriculture and industry. 煤能够用来为工农业生产发电。

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