can 分两种表能力时 前面肯定则后面用否定 can't；前面 否定则后面用肯定can 表猜测时 前面是肯定后面用 be动词否定形式或者助动词否定形式+主语.反之亦然.如 It can be right,isn't it?It can happen soon,doesn't ...