1它们不能重复使用,,即不能说 can be able to
2can表示本能；be able to 表示通过努力才能做成某事
3.can只有两种时态,现在时can和过去时could
be able to 有多种时态的变化,,
如.1.The baby can walk now (本能）
2.His sick grandfather is able to walk by himself now (通过身体恢复,才能够走路的）
3.I have been able to speak five kinds of languages fluently.
4.You'll be able to see him tomorrow.