be able to 强调通过努力而获得的能力,而can则强调自身已具有的能力。be able to 强调一种结果,而can只强调一种可能。be able to 可以有各种时态;而can只有一般现在和一般过去两种时态。can可用于表示可能性,推测,允许等情况,而be able to通常不这样用。
1、She can sing the song in English.她能用英语唱这首歌。
He will be able to sing this song in English in a few minutes, too.几小时之后,他也能用英语唱这首歌。
2、Luckily, he was able to escape from the big fire in the end.幸运的是,他终于逃出了大火。
If he got here a few hours earlier, I could save him.要是他早几小时来,我还能救他。
3、I could help you last night, but you didn’t come.昨天晚上我能帮你,而你又没来。
Can you see it there?你能看见它在那儿吗?
He is / was / will be able to help you.他能帮你的忙。
这一点不同主要集中在过去时中,could 表示“能够”,was / were able to 表示“得以做成”,即后者的动作完成了,前者只是一种可能性。
He could leave Europe before the Second World War. 他在二战开始前能离开欧洲。(有没有离开,不知道)
He was able to leave Europe before the Second World War. 他在二战开始前得以离开欧洲。(成功离开)