1. be able to 和 can 意思相近, 都可以泛指现在或过去的某种能力时，一般互换，但 be able to

   可有更多的时态形式。

   Can you speak English? = Are you able to speak English? 你会说英语吗？

   He could (=was able to) swim when he was ten years old. 他十岁时就会游泳。

   We shall be able to finish the work tomorrow. 我们可以在明天完成这工作。

   He hasn't been to come. 他没有能来。

 

2. be able to 表示设法做成了某事, 侧重结果；can 侧重自身能力

   There was a big fire and only 3 people were able to get out.

   I could run after a bus and catch it twenty years ago, but I can't do that now.

 

3. be able to 可以和情态动词或某些系动词连用;  can 则不能。

   I ought to be able to live on my salary.

   You might be able to persuade him.

 

4. be able to 也可用于不定式或动名词短语中;  can 则不能。

   He said he so much regretted not being able to swim.