.定语  修饰和限定名词和代词的词。通常由形容词，代词(多为形容词性物主代词），数词，介词短语，动词不定式，分词和从句（定语从句）来担当。
    a kind girl,
    a book on the desk
     I have homework to finish.
     the fallen leaves
     the boy who is very smart

定语从句相当于一个形容词，跟在名词或代词后修饰该名词或代词。

1. 被定语从句修饰的名词或代词叫先行词。
2. 引导定语从句的词叫关系词。
    关系代词：who, whom(作宾语成分）, whose,          that,  which
     关系副词：when（时间）, where（地点）,why（原因）
关系代词和关系副词的作用：
1、引导作用    2、替代作用
3、在定语从句中担当某个成分的作用
Eg: 1.Those who want to go please  should be ready.
2. Bill, who was my student , asked me questions.

关系词引导定语从句，同时指代先行词在 定语从句中充当的成分
   所以先行词在定语从句中不再出现,也不用其他词代替
关系副词引导定语从句时，既引导定语从句，又在定语从句中充当状语。不能省略.
关系副词可转化为介词+关系代词结构。
1、When指时间，在定语从句中充当时间状语。
      He told me the date when(on which) he joined the Party.
2、Where指地点，在定语从句中充当地点状语。
      This is the place where(in which) we lived ten years ago.
3、Why指原因，在定语从句中充当原因状语。
      I know the reason why(for which) she was so angry.
     
同样是修饰一个地点，有时使用where，有时使用that/which；同样是修饰一个时间，有时使用when， 有时使用that/which。这主要看两点：一：先行词在从句中所作成分；二：是定语从句中的谓语动词是否是一个及物动词。
五.一般说来,定语从句总是紧跟在先行词的后面,但有时候定语从句与先行词之间被其他成分分隔开来,这样的句子叫分隔定语从句.
There are thousands of stars in the sky that are like our sun.
The days are gone when we suffered so much.
Carl still remembers one afternoon in his first year when the professor took the students to the chemistry lab.
   as可作为关系代词来引导定语从句。
such … as…“像……一样的”，“像……之类”
      the same…as…“和……同样的
      在这两个句型中，as是关系代词，其中such&same做定语，修饰主句的名词、代词，这个名词是这个定语从句的先行词，as在从句中担任主、宾、表。
例如：This is the same thing as we are in need of.
      Such people as you say are short now.
2. …such as…
    such为代词，意思为”这样的人/物“，as修饰先行词such。
   例如：This book is not such as I hope.
3. the same…as…    “和…同样的 “
    the same… that…  “同一个”
    来引导定语从句。意思几乎相同。但从句中省去谓语时必须用as.
 例如:She is the same age as you (that you are.)
 This is 以下情况，引导词用that，不用which
1. 先行词为不定代词或被不定代词所修饰everything，little，much，all，anything，nothing,etc.
e.g. Finally, the thief handed everything that he had stolen.
2. 先行词前有形容词最高级、序数词修饰时，用that。
e.g. This is the best book (that) I’ve ever read.
the same pen that I lost yesterday.
4. …,as…   “这些”，“正如…”
    as引导非限定性定语从句，相当于which引导的非限定性定语从句，有时可互换。
例如：He is from Beijing, as/which they know from the way he speaks.

3. 先行词被the only，the very，the last 修饰时，用that。
e.g.He is the only person that I want to see now.
4. 先行词同时指人和指物时，用that。
e.g. We talked about the things and persons that we saw then.
5. 在who, which引导的特殊疑问句中, 定语从句中避免重复,一般也只用that; e.g. Who is the man that is crying?
6.关系代词在从句中做表语，用 that.
He doesn’ seem to the man (that)he was.
7.在 there be结构中用  that
There is a new type of shoes that you might be fond of.
1.Anyone who breaks the law is punished.
2.Those who break the law are punished.
3.He who breaks the law is punished.
4.He who has not been to the Great Wall is not a true man.
1. Which.as引导非限定语从句的不同：
as引导非限定语从句可以放在句首，句中，或主句后面； which引导非限定语从句放在主句后面
  Eg:As is known to all,Taiwan belongs to China.
      Mary was late again ,which made the teacher angry.
2. as引导非限定语从句有“正如”“ 正象”之义，而 which 没有,常见的结构有：
As we all know,as everybody can see, as is known to all, as is said above…
 Paper, as we all know,was first made in China.

1. what不引导定语从句.what=all that
   I want to give you what I have.(宾语从句）
2. 先行词被the same 修饰时，关系代词既可用that, 也可用as.但意义不同，前者表示“同一的”，后者表示“同样的”。
3. 关系副词when/where/why其含义相当于on which/in which/for which等可交替使用。如：The day when/on which I met him first was May 1st.
4.当先行词为专有名词或整个句子是世界上独一无二的名词时，一般用非限定性定语从句修饰。
   例如：The sun, which gives us light, is very big.