这两个词的用法和过去时无关,主要还是要看具体意思,一般来说,can的意思多些,有“能、会、可以”等意思,而be able to的意思一般表示“有能力做……”,在此基础上,在时态上也有区别,一般现在时态和一般过去时用法一样,表示“能……”,但在将来时和完成时或与其它情态动词连用时,就只能用be able to了,换句话说, be able to 在时态变化上比较灵活。如:
You can come if you like. (be able to不行)
He is able to work efficiently. (can也行)
He should be able to come earlier.
He has been able to solve such problems.
They will be able to do that.
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