1.定语从句that作主语不能省略; Which is the book that was borrowed from the library? that三个作用: 1. 引导限制性定语从句; 2. 从句中做主语; 3. 有意义,指代先行词book. 2.as引导定语从句从句中做主表宾。 As we all know, Lu Xun is a great writer in China. as 三个作用: 1。引导非限制性定语从句; 2. 从句中做及物动词know的宾语; 3. 有意义,指代后面整个主句的内容。