1）can could 表达才能；可能 (过往时用could),
只用于如今式和过往式(could)。be able to能够用于各类时态。
They will be able to tell you the news soon。
他很快就能告诉你动静了。
2）只用be able to
a。 位于助动词后。
b。 情态动词后。
c。 表达过往某时刻动做时。
d。 用于句首表达前提。
e。 表达胜利地做了某事时，只能用was/were able to， 不克不及用could。
He was able to flee Europe before the war broke out。
= He managed to flee Europe before the war broke out。
重视：could不表达时态
1）提出委婉的恳求，（重视在答复中不成用could）。
--- Could I have the television on?
--- Yes, you can。 / No, you can't。
2）在否认，疑问句中表达揣度或思疑。
He couldn't be a bad man。
他不大可能是坏人。
can用于表达“才能”时是指如今的才能，过往或未来的才能凡是用was/were able to或will/shall be able to表达。
eg。 She was able to go to school yesterday。
Soon she will be able to swim a quarter of a mile。
Can的过往式形式could也可用于表达泛指过往的才能。如：
I could read when I was four。
但不克不及用于表达特定的某一过往才能，如不克不及说：
*He could swim halfway before he got tired。
He was able to swim halfway before he got tired。
上述那种用法的区别不存在于否认句中。在否认句中，couldn’t与was/were not able to 能够互换利用。
eg。 I couldn’t see him again before he left。
I wasn’t able to see him again before he left。
别的，要说一个阶段延续至今的才能，可用have/has been able to表达。
eg。 John has been able to swim for many years。
重视的一点是：can也能够用于表达“答应”。
它能够表达如今答应或未来答应，过往答应用could表达，此时一律不克不及用be able to的某种形式替代。