先行词不可缺少的定语，如删除，主句则失去意义或意思表达不完整。
B.非限制性定语从句只是对先行词的补充说明，如删除，主句仍能表达完整的意思。译法上译成先行词的定语?．．．的? 通常译成主句的并列句。 在非限制性定语从句中，通常用who, whom, whose 指人，用 which 指物。
Eg. Her brother who is now a soldier always encourages her to go to college.
她那当兵的哥哥总是鼓励她上大学.（意含：她还有其他哥哥。）
Her brother, who is now a soldier, always encourages her to go to college.
她哥哥是当兵的，他总是鼓励她上大学。（意含： 她只有一个哥哥。） 五大考点：
考点一. 先行词为人时引导词who和that a. 用who不用that的情况：
(1)当先行词为one, ones, anyone, those时。(2)当先行词为人称代词时。
He who does not reach the Great Wall is not a true man.
Those who have any difficulty with pronunciation should practice more.
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b. 用that不用who的情况：
1)当主句已经出现who时。2)关系代词在定语从句中作表语时。 Who is the man that spoke to you just now? Mary is no longer the girl that she used to be. 考点二. 先行词为物时引导词that和which a.只用that不用which 的情况：
(1)先行词为much, little, few, nothing, none, anything, no, all等不定代词 （2）先行词既有人又有物。
（3）先行词被形容词的最高级或序数词修饰。
（4）先行词被the very, the only, the last, just, all, any, every, no等修饰
（5）关系代词在从句中作表语．
(6) 在疑问词who、which、what开头的句子中 (7) 主句是there be句型：
This is one of the best films that I have ever seen. China is no longer the country that it used to be. All that can be done has been done.
He spoke of the things and persons that he had seen abroad. This is the very book that I am looking for. Who is the man that is talking to Jim? There is a man that lives in that village.
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b.只能用which不用that的情况: 1.介词后面 2.逗号后面 1. 定语从句中的介词前置时关系代词只能用which 2. 引导非限制性定语从句时只能用which, 其先行词可是一个词,也可是整个主句或主句的某一部分.
(1) The machine, which I have looked after for many year, is still working perfectly.
(2)My glasses, without which I was like a blind man, fell to the ground and broke.
考点三：★★★★★介词+关系代词 (which/ whom) **** 关系代词前介词的确定方法：
1. 定语从句的动词与先行词的逻辑关系，或者从句的动词、形容词的习惯性搭配。
Eg. The farm on which I once worked has taken on a new look Who is the man with whom our teacher is shaking hands? Ours is a beautiful country, of which we are greatly proud. 2.非限制性定语从句中，表示先行词的一部分时，可用“数词/代词 + of + 关系代词”的结构，
e.g. There are 50 students in our class, two- thirds of whom have been to Beijing.
3. Whose从句可转换为“ of +关系代词?型，whose的先行词指物时, 可用of which代替,
但词序不同， 即whose+n=the +n + of which =of which + the + n.
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e.g. They live in a house, whose door opens to the south.
They lived in a house, the door of which open to the south. Of which the door open to the south. 考点四:关系副词的运用
⑴ 先行词为“时间的名词”用when：time（day, night, morning, week, month, year等）＋when…
Eg. I don?t remember the day when he left.(on which) ⑵ 先行词为“表示地点的名词” ★★★★★ where：place（case, point, situation, condition, scene, stage, country, room, house, hotel, museum, school, street等）＋where…
Eg. That is the house where he lived two years ago. (In which) ⑶ 先行词为“表示原因的名词”why：reason＋why…（表示原因的名词只有一个）
That's the reason why I helped him.（why =for which） 考点五：★★★★★ as与which引导的定语从句
两者均可引导非限制性定语从句，as 放在句首句中都可以，which 必需放在句中，但下列情况多用as。
1. 关系代词引导的定语从句居句首时。e.g. As we all know, the earth is round.
2. 当与such , as或the same连用时，一般用as。 e.g. That?s the same tool as I used last week.
I am not so strong a man as I was.我已经没有从前那么强壮了。 3. 当主句和从句之间存在着逻辑上的因果关系时，关系词往往只用which。
Eg.Tom was late for school again and again, which made his teacher very angry.
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三大区别：
1.the same... as 和 the same ...that的用法辨异.
That?s the same tool as I used last week.（同类事物）
That?s the same tool that I used last week. 那就是我上周用过的工具。（指原物） 2. 定语从句与强调句型的区别 判断是否是强调结构的方法是采用删除法
分析：强调句句型：It is/was +被强调的成份+that/who +其它部分．去掉 It is/was….that/who… ,句子照样成立。
①It is on the morning of May 1st _____ I met Liang Wei at the airport．
②It is the factory _____ Mr Wang works． 3、定语从句与习惯句型的区别
①It is the first time _____ she has been in Shanghai． （It is／was the first／second ． time ＋that从句） ②It was the time _____ Chinese people had a hard life． 三大注意： 1. the way 做先行词时，定语从句可由that, in which 引导或不用引导词。
2. 定语从句中主谓一致问题：从句中的动词在人称和数方面应该与它的先行词保持一致。
He is one of the teachers who know English well. He is the only one of the teachers who knows English well.(特殊) 3.疑问句的解题思路 ：先把疑问句还原成陈述句，然后判断谁是先行词，再看关系词在定语从句中所充当的成分，最后确定正确答案。
1. Is this factory ____ we visited last week? 2. Is this the factory ____ we visited last Week?
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