（二）关系代词as和which引导的定语从句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。
(1) He married her, as/which was natural.
(2) He was honest, as/which we can see.
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。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.
3、当先行词受such, the same修饰时，常用as
(1) I have never heard such a story as he tells.
(2) He is not such a fool as he looks.
(3) This is the same book as I lost last week.
注意：当先行词由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
(4) She wore the same dress that she wore at Mary’s wedding.
她穿着她在MARY婚礼上穿过的一条裙子。
(5) 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。
注意：定语从句such…as …与结果状语从句such… that…的区别：as在所引导的定语从句中作主语，宾语；that在结果状语从句中不做成分
(6)He has such a good laptop as I want to buy.
(7)He has such a good laptop that I want to buy one.
(三）以the way为先行词的定语从句通常由in which, that引导，而且通常可以省略。
The way in which/ that/./ he answered the question was surprising.
(四) but有时也可以做关系词引导定语从句
There are very few but understand his idea。 ( but= who don’t )
(五) 区分定语从句和同位语从句
1、定语从句修饰先行词，它和先行词是修饰关系；
同位语从句说明先行词的具体内容，是补充说明的关系
(1) The plane that has just taken off is for London. 定语从句
(2) The fact that he has been dead is clear. 同位语从句
2、定语从句由关系代词或者关系副词引导，关系词在句中充当成分，有时可以省略；
同位语从句主要由that引导，在句中一般不做成分；
句子也可以由when, where, how, why, whether, what等词引导，充当成分
(1) The news he told me is true.
(2) The news that he has just died is true.
(3) The problem that we are facing now is how we can collect so much money. 定语
(4) The problem how we can collect so much money is difficult to solve.
3、 同位语从句和先行词一般可以用be动词发展成一个完整的句子，而定语从句不可以发展成一个完整的句子，
(1) The idea that we could ask the teacher for advice is wonderful. 同位语
(2) The idea was that we could ask the teacher for advice.
(3) The fact that the moon moves around the earth is known to all.
(4) The fact is that the earth moves around the earth.
[定语从句]介词+关系词
1）介词后面的关系副词不能省略。
2）that前不能有介词。
3）某些在从句中充当时间，地点或原因状语的"介词+关系词"结构可以同关系副词when 和where 互换。例如：
This is the house in which I lived two years ago. 这是我两年前住过的房子。
This is the house where I lived two years ago.
Do you remember the day on which you joined our club？ 还记得你加入我们俱乐部的那一天吗？
Do you remember the day when you joined our club？