众所周知, 定语从句可以分为限制性定语从句和非限制性定语从句。下面结合考例谈谈学习运用非限制性定语从句时应注意的几个问题：
一、that不能用来引导非限制性定语从句。如：
【考例一】She heard the terrible noise, ________brought her heart into her mouth.
A. it B. which C. this D. that
【解析】选B。后半句为非限制性定语从句，尽管先行词noise属于事物，也不能用that而要用which来引导。which指代noise，在从句中作主语。
二、除which外，还可用when，where，whose，whom等引导非限制性定语从句，且各自的用法及差异与它们用在限制性定语从句时相同。如：
Next month, when you'll spend your summer holidays in your hometown, is approaching. 下个月即将来临，届时你们将在家乡度暑假。(关系副词when指代表示时间的名词next month，并作从句的状语。)
She is going to live in Macao, where she has some close friends. 她要到澳门去居住，在那里她有几个密友。 (关系副词where指代表示地点的名词Macao，并在从句中作状语。)
【考例二】Recently I bought an ancient Chinese vase, ________was very reasonable.
A. which price
B. the price of which
C. its price
D. the price of whose
【解析】选B。whose在从句中作表示所属关系的定语，既可指代人，也可指代物。指物时，常可转换为of which；指人时常可转换为of whom。
【考例三】 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
【解析】 先行词5:30 pm与time之间是一种同位关系，可用which来作定语替代。从句中的完成状态，常与by短语连用。故选D。
三、在非限制性定语从句中作宾语的关系代词不能省略。如：
He was eager to go to the hospital to see his stepmother, whom he loved as his own mother. 他迫切地想到医院去看望他的继母，他把继母当自己的亲妈妈一样爱戴。
四、非限制性定语从句的关系代词which，既可以指代前面的先行词，也可以指代前面整句的含义。如：
【考例四】 Dorothy was always speaking highly of her role in the play, of course,________made the others unhappy.
A. who B. which
C. this D. what
【解析】选B。指代“Dorothy总是高度评价自己在那场戏中所担任的角色”的整个事件。
【考例五】Carol said the work would be done by October,________personally I doubt very much.
A. it B. that
C. when D. which
【解析】表示时间的名词October，用“我个人非常怀疑”来修饰，意思不通。应该修饰整个主句， 表示对“Carol说在十月前能做好这项工作”怀疑。故选D。
五、引导非限制性定语从句时as和which的差异：从句置于句首时，非限制性定语从句只能用as引导，而置于句末时，两者都可以使用。 如：
【考例六】________is known to everybody, the moon travels round the earth once every month.
A. It B. As C. That D. What
【解析】逗号表明为非限制性定语从句。选B。
【考例七】________is mentioned above, the number of the students in senior high schools is increasing.
A. Which B. As
C. That D. It
【解析】答案为B。与考例六同理。
文/彭昊虹