定语从句是高一英语的主要语法项目之一，也是高考英语的常考知识点，尤其是as和which引导的定语从句，更是历年高考试题中的考点和热点。as和which都能引导定语从句，那么它们的共同点和区别是什么呢?

　　一、as和which都能引导的定语从句

　　当非限定性定语从句置于句末时，as和which都可用来引导这个定语从句。例如：

　　Dorothy was always speaking highly of her role in the play, _______ , of course, made the others unhappy. (NMET2000)

　　A. who B. which C. this D. what

　　【简析】答案是B。which引导非限定性定语从句，指代前面主句的内容，定语从句部分意为：这当然使别人不高兴。这里不能换用as作引导词，因为我们不能说“正如使别人不高兴”。

　　The number of senior high school students is increasing, _______ is mentioned above.

　　A. that B. it C. such D. as

　　【简析】答案是D。as引导非限定性定语从句，意思是“正如”。此时也可以换用which引导，但意思有别，意为：这一点上面提到了。

　　二、只能用which引导的定语从句

　　1. 在限定性定语从句中。例如：

　　The project _______ a British businessman has invested in is under construction at present.

　　A. which B. as C. what D. why

　　【简析】答案是A。单个词引导限定性定语从句时，一般不用as。当然，本题中的定语从句也可以使用that引导；由于关系词在从句中作宾语，which和that都可以省略。

　　2. “介词+关系代词”引导定语从句时，只能用which。例如：

　　Recently I bought an ancient Chinese vase, _______ was very reasonable.(上海2000高考)

　　A. which price B. the price of which C. its price D. the price of whose

　　【简析】答案是B。此时的which不能用as来代替。另外还应注意：引导定语从句时，which不能单独作定语，此时不管先行词是人还是物，都只能用whose引导，但指物时，可以与of which替换(如本题)。

　　3. 当后置的非限定性定语从句是否定结构时，只能用which引导。例如：

　　The result of the experiment was very good, _______ we hadn't expected.(2000京皖春招)

　　A. when B. that C. which D. what

　　【简析】答案是C。由于非限定性定语从句是否定结构，只能用which引导，不能用as来替代which。

　　三、只能用as引导的非限定性定语从句

　　1. 置于句首时，非限定性定语从句只能用as引导。例如：

　　_______ is known to everybody, the moon travels round the earth once every month. (NMET2001)

　　A. It B. As C. That D. What

　　【简析】答案是B。As在定语从句中作主语，这里不能换成Which。

　　2. 在the same...as 结构中，意思是“像……那样的”。例如：

　　Chongqing is not the same city _______ we have seen in other places.

　　A. that B. which C. 不填 D. as

　　【简析】答案是D。as在定语从句中作have seen的宾语。the same ... that的意思是“正是……；就是……”。注意：as不能省略。

　　3. 在as...as结构中，意思是“像……那样的”。例如：

　　No period in history has had as many important changes _______ have taken place in the past century.

　　A. which B. as C. that D. what

　　【简析】答案是B。as引导定语从句并在从句中作主语。

　　小试牛刀：

　　1. _______ has been announced, we shall have our final exam next month.

　　A. That B. As C. It D. What

　　2. The weather turned out to be very good, _______ was more than we could expect.

　　A. what B. which C. that D. if

　　3. In the office I never seem to have time until after 5:30 pm, _______ many people have gone home.

　　A. whose time B. that C. on which D. by which time

　　4. Mathilde had to work hard day and night and bought such a diamond necklace _______ she had lost to pay back to Jeanne.

　　A. as B. that C. which D. 不填

　　5. —Robert, is this yours?

　　—Yes, it's just the same Walkman _______ I lost on the playground yesterday.

　　A. which B. what C. as D. that

　　Key: 1—5 BBDAD

　　(文/陈觉法； 英语辅导报高一版 04~05学年度第9期；版权归英语辅导报社所有，独家网络合作伙伴新浪教育，未经许可，不得以任何形式进行转载。)