【同中求异】be able to与can

be able to指具有做某事的能力，常强调通过努力获得的能力，强调结果，它可用于各种时态。如：

  I’m able to swim and I like swimming very much.

  我会游泳而且我非常喜欢游泳。

  Lily is able to drive and she drives very carefully.

  莉莉会开车而且她开得很小心。

  Believe in yourself! I think you’ll be able to pass the exam.

  相信你自己！我想你会通过考试的。

My brother was able to drive five years ago.

我弟弟五年前就会开车。  

Now you should be able to speak English well.

现在你应该会把英语说好的。

My sister has been able to play the piano since two years ago.

自从两年前我妹妹就能够弹钢琴了。

can常指自身具有的能力，强调可能性，无人称和数的变化。其只能用于一般现在时（can）和一般过去时（could）两种时态。如：

She can play the piano.

她会弹钢琴。

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 he finish the work on time?

他能按时完成这项工作吗？