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。
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