They don't use more water than (it is) necessary.

　　他们使用的水没有超出需要量。

　　He runs as fast as Bob (runs)．

　　他和鲍勃跑得一样快。

　　[名师点津]　并非所有的状语从句都可以省略主语和be动词，由after, before, because等词引导的状语从句一般要改写成介词短语等，用动名词代替be动词。

　　Because he was ill, he didn't attend the meeting.

　　→Because of being ill, he didn't attend the meeting.(正)

　　→Being ill, he didn't attend the meeting.(正)

　　→Because ill, he didn't attend the meeting.(误)

　　由于生病了，他没有出席会议。

　　[即时演练3]

　　(1)用所给词的适当形式填空

　　①When surfing (surf) the Internet, I downloaded the film.

　　②Video games can be a poor influence if left (leave) in the wrong hands.

　　③He shook his head as if to_say (say) "no"．

　　(2)把下列句子改为省略句

　　①If it is so, I hope you will have a wonderful time.

　　→If_so,_I hope you will have a wonderful time.

　　②The winters in Hangzhou are not so mild as they are in Guangzhou.

　　→The winters in Hangzhou are not so mild as_in_Guangzhou.

　　2．定语从句的省略

　　(1)在限制性定语从句中，作宾语用的关系代词whom, which, that可省略(但whom, which紧跟在介词后时不能省略)。

　　The exact year(which/that)Angela and her family spent together in China was 2008.

　　安哥拉和她的家人一起在中国度过的那一年是2008年。

　　(2)修饰way的关系词that/in which可以省略。

　　The way he speaks to us is really annoying.

　　他对我们讲话的方式真是让人讨厌。

　　[即时演练4]　写出下列句中可以省略的部分

①Among the many dangers which sailors have to face, probably the greatest of all is