一定义：定语从句是由关系代词和关系副词引导的从句，其作用是作定语修饰主句的某个名词性成分，定语从句分为限定性和非限定性从句两种。
二定语从句两大要素：
1. 先行词： 被修饰的名词或代词，非限制性定语从句中，可以是整个主句充当先行词。
2. 关系词：引导定语从句的连词称为关系词。
关系词有关系代词和关系副词。
关系词常有3个作用：1. 定语从句。2.替先行词。3. 语从句中担当一个成分。
三如何选用适当的关系词：
1. 先看先行词（什么样的名词---人，物，地点，时间，原因）
2. 关键看定语从句是否是个完整的句子。①如果缺少主语，宾语，表语就要用关系代词（缺少定语就要用whose）②如果句子是完整的话，就要用关系副词充当从句中的状语。
1) i still remember the dayswe spent together.
i still remember the dayswe worked together.
2) do you still remember the placewe visited yesterday?
do you still remember the placewe used to work together?
3) this is the reasonhe g**e to us .
this is the reasonhe was late for school.
四介词+关系代词(which/whom) 引导的定语从句。
关系代词在定语从句中做介词宾语时，从句常由介词+关系代词引导。
1)the school (that/which) he once studied in is very famous.
the school in which he once studied is very famous.
2)we’ll go to hear the famous singer (whom/that/who) we h**e often talked about.
we’ll go to hear the famous singer about whom we h**e often talked.
注意：1. 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。
1)this is the watch which/that i am looking for. (t)
2)this is the watch for which i am looking. (f)
2. 若介词放在关系代词前，关系代词指人时用whom，不可用who或者that；指物时用which，不能用that；关系代词是所有格时用whose
1) the man with whom you talked is my friend. (t)
the man with who/that you talked is my friend. (f)
2) the plane in which we flew to canada is very comfortable. (t)
the plane in that we flew in to canada is very comfortable.(f)
3. “介词+关系代词”前可有some, any, none, both, all, neither, most, each, few等代词或者数词。
1) he loved his parents deeply, both of whom are very kind to him.
2) in the basket there are quite many apples, some of which h**e gone bad.
3) there are forty students in our class in all, most of whom are from big cities.
五．whose通常指人，也可指物，在定语从句中做定语。
1) he has a friend whose father is a doctor.
2) i once lived in a house whose roof has fallen in.
whose指物时，常用以下the +n+ of which/ of which the + n来代替。
the classroom whose door is broken will soon be repaired.
the classroom the door of which is broken will soon be repaired.
the classroom of which the door is broken will soon be repaired.
六．限制性定语从句和非限制性定语从句。
限制性定语从句非限制性定语从句。
形式上： 不用逗号和主句隔开用逗号和主句隔开。
意义上： 是先行词不可缺少的定语，不能删除 ; 是对先行词的补充说明，删除后意思仍完整
译法上： 翻译成先行词的定语，“…的通常翻译成主句的并列句。
非限制性定语从句举例：
1)his mother, who loves him very much, is strict with him.
2)china, which was founded in 1949, is becoming more and more powerful.
3) she is going to live in beijing, where she has some close friends.
4) the meeting will be put off till next week, when those people will be back from shanghai.
5) he has two sons, one of whom is a doctor.
6) there are two buildings in our school, the higher of which is a teaching building.
7) an big earthquake broke out, which caused great damage.
8) as is often the case in the other countries, people prefer to h**e more equal rights.
难点分析。一）限制性定语从句只能用that的几种情况。
1．当先行词是anything, everything, nothing (something 除外), few, all, none, little, some等代词时，或者是由every, any, all, some, no, little, few, much等修饰时。
1) h**e you taken down everything that mr. li has said?
2) there seems to be nothing that seems impossible for him in the world.
3) all that can be done has been done.
4) there is little that i can do for you.
注意：当先行词指人时，偶尔也可以用who
any man that/.who has a sense of duty won’t do such a thing.
2. 当先行词被序数词修饰。
1) the first place that they visited in london was the big ben.
3. 当先行词被形容词最高级修饰时。
1) this is the best film that i h**e seen.
4. 当形容词被the very, the only 修饰时。
1) this is the very dictionary that i want to buy,2) after the fire in his house, the old car is the only thing that he owned.
当先行词指人时，偶尔也可以用who
3) wang hua is the only person in our school who will attend the meeting/
5. 当先行词前面有who, which等疑问代词时。
1) who is the man that is standing there?
2) which is the t-shirt that fits me most?
6. 当先行词既有人，也有动物或者物体时。
1) can you remember the scientist and his theory that we h**e learned?
7. 定语从句中缺少表语时，用that, 不用which/who/whom
1) china is no longer a country that she used to be.
2) she is no longer a girl that she used to be.
二）关系代词as引导的定语从句。
1. 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。另外，as有“正如……，正像……”的意思。
1) as is known to all, china is a developing country.
2) he is from the south, as we can see from his accent.
3) john, as you know, is a famous writer.
4) 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which
5) tom was always late for school, which made his teacher angry.
2. 当先行词受such/so 修饰时候用 as （as 在定语从句充当主语/宾语）引导定语从句。
i h**e never heard such a story as he tells.
比较：(1) this is such a he**y box as can’t be moved easily. (as是关系代词在定语从句中充当主语。)
2) this is such a he**y box that it can’t be moved easily.
that 后面从句中没有缺少主语/宾语，所以这个that 不是关系代词引导定语从句，而是一个结果状语从句， 如此…以致于…)
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