can和 be+形容词 able在此互相关联;后者补充 can所没有的形式,并可以替换can用于现在时和过去时,因此有以下各种形式:
不定式:to be able
过去分词: been able
将来时只有一个形式,因为can除了表示许可时外不能用于将来时,但是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,都按照普通动词的规则来构成:
现在完成时:have been able
过去完成时:had been able
否定疑问式:could you not?/couldnt you?were you not?/werent youable?will you not?/wont you be able?等
can/be/will/shall not和 have都可以按通常方法缩略:
I wasnt able he wont be able Ive 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。
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