can 与 be able to 均可表示某人做某事的能力，常可互换。例如：
I'm not able to [can't] explain this. 我不能解释这个问题。
He was able to [could] speak French very well. 他的法语讲得很不错。
但是两者还是有区别的：
(1) 从时态形式来看，can 只有现在式和过去式(could)两种，而 be able to 则可根据需要使用多种时态，还可以和一些情态动词连用，或用作动词不定式等非谓语形式。例如：
I haven't been able to sleep recently. 最近我睡不着觉。
Our baby will be able to walk in a few weeks. 我们的宝宝再过几个星期就能走路了。
You might be able to persuade him. 你也许能够说服他。
I hope to be able to do the work. 我希望能干得了这项工作。
I regret not being able to help her. 我很遗憾未能帮助她。
(2) 用于现在时，can 泛指一般的能力，而 be able to 则主要指具体做某件事的能力。例如：
He can swim. 他会游泳。
I am able to express my idea in a clear way. 我能清楚地表达自己的想法。
(3) 在表示过去的一般能力时，两者没有很大差别。例如：
I could / was able to play the piano when I was young. 我年轻时会弹钢琴。
但如果要表示"一时的能力"，即"在过去某时有某种能力，并顺利地完成某事"，则要用 was / were able to，而不能用 could。例如：
Though he was sick, he was able to swim across the river. 虽然他病了，但他还是游过了河。
After five hours of climbing, we were able to reach the top of the mountain. 经过5个小时的攀爬，我们终于登上了山顶。
could 表示可能或有能力做，was [were] able to 则表示能够做且确已做到。比较下面两句：
I was able to pass the examination. 我通过了考试。(不仅有能力，而且实际上做到了。相当于I succeeded in passing the examination.)
I could pass the examination. 我当时有能力通过考试。(有能力，但实际上不一定做得到)