①could have done表示对过去所发生事情的推测,一般用于否定句和疑问句。 He couldn't have left for Beijing, for I saw him just now.他不可能去北京了,因为我刚才看见他了。 Could he have told his mother the truth?他会把真相告诉他母亲吗? ②could have done本...
1)can could 表示能力;可能(过去时用could), 只用于现在式和过去式(could)。be able to可以用于各种时态。They will be able to tell you the news soon.他很快就能告诉你消息了。 2)只用be able to a. 位于助动词后。 b. 情态动词后。 c. 表示过去某时刻动作时。
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