1) can和could表示能力，can用于现在时，could用于过去时或表示委婉的说法，而be able to可以用于各种时态。

　　【例如】He is over 80 but still can read without glasses.

　　He could speak French, so he was able to help the French lady yesterday.

　　This is the information I've been able to get so far.

　　I used to be able to remember thing quickly.

　　2) can和could可以表示可能性和许可。在表示许可时，may比较正式。

　　【例如】

　　Sports and games can be fairly expensive. (可能性)

　　You can walk for miles in the country without meeting anyone. (可能性)

　　You can go now if you really have something urgent. (许可)

　　You may return the books you borrowed to the library on Friday. (许可)

　　3) can和could接动词的完成形式，表示可能已经做某事。can用在否定和疑问句中，表示

　　不相信、怀疑等态度。

　　【例如】

　　They can't have gone out because the light is on.

　　He can't be in the library, for I saw him in the department store a moment ago.

　　You ____her in the office last Friday; she has been out of town for two weeks. (CET-4 1996, 1)

　　A) needn't have seen B) must have seen

　　C) might have seen D) can't have seen

　　本句译为：你上星期五不可能在办公室看到她。她已经离开镇上两个月了。can't have seen意思是：不可能干什么，含不相信的态度，与句意相符，故选择D.