Ⅰ. can “能”表过去或现在“能力所及”时，与be able to 通用。如：
① He can / is able to speak German. 他会说德语。
② He could / was able to run very fast when I was a boy. 我小时候就跑得很快。
Ⅱ. be able to 的过去时还可表一种“经过努力做到了”的意思。而can的过去时则没有这种意义。如：
He started late, but he was able to catch the eight o’clock train. 他出发晚了，但他还是赶上了八点钟的火车。
Ⅲ.can 通常只用于上述两种时态，而be able to 则可用于各种时态。如：
He has not been able to come since he got hurt. 自从受伤以后，他就一直未能前来。
Ⅳ. 在表猜测时，只能用can，而不能用be able to ，如：
That can’t be your bag. 那不可能是你的书包。