2.do替代主动词的用法

do还可以只替代主动词而不替代其附带成分。

A feather falls slower than a stone only because the air holds the feather back more than it does(=holds back)the stone．

羽毛比石头落得快，只是因为空气对羽毛的阻力要比对石头的大。

She plays the guitar better than she does(=plays)the piano．

她的吉他弹得比钢琴好。

Perhaps if this particular student had spent as much time studying as he did(= spent)writing on his clothes，he would have passed．(人教大纲版高三第239页)

假如这个学生花在学习上的时间和花在衣服上写(作弊答案)的时间同样多的话，他或许已经通过了(科举考试)。

However，we actually learn a foreign language much faster than we do(=learn) our mother tongue．(人教大纲版高三第67页)

然而，事实上我们学习外语比我们学习母语要快得多。

He is learning Chinese，and I am doing(=learning)Japanese．

他学的是汉语，而我在学日语。