can和 be+形容词 able在此互有关联;后者补充 can所没的形式,并可以替换can用于目前时和过去时,因此有以下各种形式:
不定式:to be able
过去分词: been able
以后时只有一个形式,由于can除去表示许可时外不可以用于以后时,但在条件语气中则有两种形式:could和 would beable。
be able的所有其他时态,都根据普通动词的规则来构成:
目前完成时:have been able
过去完成时:had been able
否定疑问式:could you not?/couldn't you?were you not?/weren't youable?will you not?/won't you be able?等
can/be/will/shall not和 have都可以按一般办法缩略:
I wasn't able he wont be able I've been able
can后面带不带to的不定式(即动词原形)。
be able后面带有 to的不定式。