根据定语从句与先行词的关系，定语从句可分为限制性定语从句及非限制性定语从句。
限制性定语从句紧跟先行词，主句与从句不用逗号分开，从句不可省去；非限制性定语从句主句与从句之间有逗号分开，起补充说明作用，如省去，意思仍完整。
在以上几个单元我们已经学习了限制性定语从句，现在把两种从句作对比学习。
一、限制性定语从句1. 关系代词that 既可代表事物也可代表人，which 只代表事物，它们在从句中作主语或宾语。
在从句中作宾语时常可省略。
I was the only person in our office that was invited to the palace ba．ll The presenttha(t which )I received last week was from my siste．r This is a book which is about space rocket technolog．y2. 关系代词who 和whom 引导的从句用来修饰人，分别作从句中的主语和宾语，whom 作宾语时，要注意它可以作动词的宾语也可以作介词的宾语。
如：This is the man who helped me．The doctor whom you are looking for is in the room．3. whose是关系代词，修饰名词，作定语，相当于who和whom的所有格，既可以修饰人又可以修饰物。
如：Do you know the student whose name is Wang Fe？iWe lived in a room whose window opens to the west.4. where是关系副词，用来引导表示地点的定语从句，where在从句中作状语。
如：At last we arrived at a small village where we ' ll work for a week.5. when是关系副词，引导定语从句表示时间，在从句中作状语。
注意，表示时间的the time，the day, the moment作先行词，其定语从句引导词有四种情况：1)when, 2) in/ on which 3) that 4) 省略。
如：By the time you arrive in London, we will have stayed there for two weeks.I still remember the day I met her.Each time he goes to business trip, he brings a lot of living necessities.6. why 是关系副词，引导定语从句表示原因，在从句中作状语。
7. 关系副词when, where, why 可以用“介词+ 关系代词”结构代替，但有两点要注意：1）在“介词+关系代词”结构中，不能用that。
关系代词指物用which，指人时用whom，不用who;且which和whom在这个结构中不能省略。
如：The plane in which we flew to Canada is very comfortable. （正确）The plane in that we flew in to Canada is very comfortable. 错（误）2）在使用介词+关系代词引导的定语从句时，含有介词的动词短语一般不拆开使用，如：look for, look after, take care of 等。
如“This is the watch which/that I am looking for.” 不可以改为“This is the watch for which I am looking.”二、非限制性定语从句非限制性定语从句的作用是对所修饰的成分作进一步说明，通常和主句间用逗号隔开，将从句拿掉后其他部分仍可成立1. which 引导的非限定性定语从句，可用来说明前面整个句子的情况或主句的某一部分。
如：She heard a terrible nois，e which brought her heart into her mouth．David studies hard and is ready to help other，swhich is what his parents expec．t2. 在非限制性定语从句中，不能用that，而用who, whom代表人，用which 或as代表事物。
关系代词作定语时用whoseo如：He loved his parents deeply, both of whom are very kind to him.In the basket there are quite many apples, which have gone bad.There are forty stude nts in our class in all, most of whom are from big citie限His mother, who loves him very much, is strict with him.China, which was founded in 1949, is becoming more and more powerful.限制性定语从句和非限制性定语从句有时表达的意义不同，试分析下面两个句子的差别：His brother who is now a doctor always encourages him to go to college. 他那当医生的哥哥常鼓励他要考上大学。
（他还有其他的哥哥）His brother, who is now a doctor, always encourages him to go to college.他的哥哥是当医生的，常鼓励他要考上大学。
（他只有一个哥哥）3. as和which引导非限制性定语从句as和which引导非限制性定语从句，有相同之处也有不同之处。
具体情况是：1) As 和which 都可以在定语从句中做主语或者宾语，代表前面整个句子。
He married her, as/which was natural.He was honest, as/which we can see.As we know, smoking is harmful to one's health.The sun heats the earth, which is very important to us.As we know, smoking is harmful to one 's health.As is know, smoking is harmful to one 's health.2) as 引导非限制性定语从句，可放在主句之前，或者主句之后，甚至可以切割一个主句；which 引导的非限制性定语从句只能放在主句之后。
As is known to all, China is a developing country.He is from the south, as we can see from his accent.John, as you know, is a famous writer.He has been to Paris more than several times, which I don 't believe.注意：当主句和从句存在逻辑上的因果关系时，常用whichTom was always late for school, which made his teacher angry.3) the same…assuch…as是固定结构,意思是“和..... 一样.... 。
”女口：I have got into the same trouble as he (has).I have never heard such a story as he tells.He is not such a fool as he looks.This is the same book as I lost last week.注意：当先行次由the same修饰时，偶尔也用that引导定语从句，但是和由as所引导的定语从句意思不同。
如：She wore the same dress that she wore at Mary 's wedding.她穿着她在玛丽婚礼上穿过的一条裙子。
She wore the same dress as her young sister wore.她穿着和她妹妹所穿的一样的裙子。
语法专项练习单项填空1. The soldier ran to the building, and __ flew a flag.A. on the top of whichB. on the top of whomC. on the top of itD. which2. When the same man _ murdered the old lady entered the hotel once more,the waiter immediately phoned the police.A. asB. thatC. whichD. whom3. He wore, __ was very common at the time, a black jacket.A. thatB. whichC. itD. what4. Was it in the room __ M r. Johnson lived ___ the exhibition was held?A. that; thatB. where; thatC. where; whereD. that; where5. _ , the population of China is the largest in the world.A. It is known thatB. As is well knownC. Which is well knownD. We all know that6. This is the very reason __ he gave me.A. thatB. whyC. for whichD. which7. He arrived in New York in 1949, _ , some time later, he became a writer.A. whenB. whereC. thatD. which8. They ' re talking about the newly opened market, ___ you get all ___ you need.A. in which; whichB. where; thatC. where; whichD. which; that9. She heard a terrible noise, __ brought her heart into her mouth.A. itB. whichC. thisD. that10. I said nothing, ___ made her angry.A. whichB. whatC. /D. that11. I shall never forget the time __ the blacks and whites were fighting side by side.A. whenB. thatC. /D. A, B and C12. He has three sons, __ are doctors.A. two of whomB. both of whomC. all of themD. each of them13. He tore up my photo, and __ made me angry.A. thatB. whatC. whichD. who14. I still remember the sitting-room __ my mother and I sat in last year.A. whereB. in whichC. thatD. to which15. The day we looked forward to __ .A. comeB. comingC. having comeD. has come答案1. C。
