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:% t+ y' g+ i- U6 N
不定式:to be able# l( S4 A- l/ ?. \8 E6 ?$ l+ s
过去分词: been able4 K7 V( z6 x' Q* R
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。# e- o( N% K' E; P6 g U: i8 O4 L( V' `
be able的所有其他时态,都按照普通动词的规则来构成:. _2 _/ N! W: U
现在完成时:have been able # {+ t" `# D" z7 E 过去完成时:had been able9 I6 B7 o, e* R
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' D; W/ ]1 |: n, @5 v' Y
can/be/will/shall not和 have都可以按通常方法缩略:# s! U: B0 C1 W& O( r- r
I wasn't able he won’t be able I've been able 7 {7 [( m4 A! W/ }" q8 i+ q can后面带不带to的不定式(即动词原形)。! Y2 m& d: n/ f9 [; J
be able后面带有 to的不定式。