can 與 be able to 均可表示某人做某事的能力，?？苫Q。例如：

I'm not able to [can't] explain this. 我不能解釋這個(gè)問(wèn)題。

He was able to [could] speak French very well. 他的法語(yǔ)講得很不錯(cuò)。

但是兩者還是有區(qū)別的：

(1) 從時(shí)態(tài)形式來(lái)看，can 只有現(xiàn)在式和過(guò)去式(could)兩種，而 be able to 則可根據(jù)需要使用多種時(shí)態(tài)，還可以和一些情態(tài)動(dòng)詞連用，或用作動(dòng)詞不定式等非謂語(yǔ)形式。例如：

I haven't been able to sleep recently. 最近我睡不著覺(jué)。

Our baby will be able to walk in a few weeks. 我們的寶寶再過(guò)幾個(gè)星期就能走路了。

You might be able to persuade him. 你也許能夠說(shuō)服他。

I hope to be able to do the work. 我希望能干得了這項(xiàng)工作。

I regret not being able to help her. 我很遺憾未能幫助她。

(2) 用于現(xiàn)在時(shí)，can 泛指一般的能力，而 be able to 則主要指具體做某件事的能力。例如：

He can swim. 他會(huì)游泳。

I am able to express my idea in a clear way. 我能清楚地表達(dá)自己的想法。

(3) 在表示過(guò)去的一般能力時(shí)，兩者沒(méi)有很大差別。例如：

I could / was able to play the piano when I was young. 我年輕時(shí)會(huì)彈鋼琴。

但如果要表示"一時(shí)的能力"，即"在過(guò)去某時(shí)有某種能力，并順利地完成某事"，則要用 was / were able to，而不能用 could。例如：

Though he was sick, he was able to swim across the river. 雖然他病了，但他還是游過(guò)了河。

After five hours of climbing, we were able to reach the top of the mountain. 經(jīng)過(guò)5個(gè)小時(shí)的攀爬，我們終于登上了山頂。

could 表示可能或有能力做，was [were] able to 則表示能夠做且確已做到。比較下面兩句：

I was able to pass the examination. 我通過(guò)了考試。（不僅有能力，而且實(shí)際上做到了。相當(dāng)于I succeeded in passing the examination.）

I could pass the examination. 我當(dāng)時(shí)有能力通過(guò)考試。（有能力，但實(shí)際上不一定做得到）