引导词是英语语法中的专有名词。引导词就是引导
从句的词，比如
whether，how，what ，why，if，when等具有引导紧贴后面从句功能的词。
定语从句
定语从句的引导词有关系代词who， whom，whose，which，that， as；
关系副词when， where，why
应用
缺少成份
定语从句中引导词的选用主要看
先行词在
定语从句中作什么成分或者是定语从句中缺什么成分。
1．定语从句中缺主语，修饰人时用who /that，修饰物时用which /that引导。如：
Is this the factory which /that makes TV sets.
The man who /that visited our school is from Australia．
2．从句中缺宾语，修饰人时用 who /whom /
that或省略引导词；修饰物时用which /that或省略引导词。如：
Gone are the days which /that we spent together in the village．
He is the man who /whom /that I think to be worthy of our praise．
3．从句中缺
时间状语，用when或介词+which引导。如：
The day finally came when /on which I was given a job．
We'll
put off the outing until next week，when we won't be so busy．
4．从句中缺
地点状语，用where或介词+which引导。如：
Please put the letter on the desk where /on which he can easily find it．
5．从句中缺原因状语或
先行词是
reason时，引导词用why /for which / that。如：
This is the reason why /for which / that he
didn't come to the meeting．
6．从句中缺定语，人和物都用 whose引导。如：
A child whose parents are dead is called an orphan．
Please pass me the dictionary whose cover is black．
7．当
先行词是way时，其
定语从句的引导词用in which /that。如：
The way in which /that he looks at problems is wrong．
as
1．在
固定词组the same．．．as，such．．．as中，as引导
限制性定语从句。如：
He is not such a fool as he looks．
I've bought the same dress as she is wearing．
2．当
先行词是整个
主句时，可用 as /which引导非限制性定语从句。引导词as和which的区别在于：
①as引导的
非限制性定语从句可放于主句前、主句中、主句后，而which引导的非限制性定语从句一般放于主句后或句中。
②as常与从句中的know，see， hear，
expect等动词连用，也常用于as often happens，as is often the case（常有的事）等句子中；而which一般不用于以上情况。
③as有“正如”的含义，which没有此含义。如：
As is well known，Shanghai is a big city．
Some of the roads were flooded， which made our journey more difficult．
非限制性定语从句
关系代词who，
whom，whose， which和as能引导
非限制性定语从句，而that不能；非限制性定语从句中即使缺宾语，引导词也不能省略；关系副词when，where能引导非限制性定语从句，而why不能。如：
Abraham Lincoln，who led the United States through the Civil War years，was shot on April 14，1865．
互换规则
先行词是物时，其引导词可用 which也可用that，通常情况下二者可互换，但在有些情况下不能。
1．用which不用that引导
定语从句的三种情况：
①在引导
非限定性定语从句时，和在介词后不能用that，应用which。如：
Football，which is a very interesting game，is played all over the world．
②先行词为that时，为了避免重复，定语从句用which不用that引导。
如：That which you borrowed from me wasn't a real diamond necklace．
③介词后用which不用that引导。如：
The method with which you solved the problem is very good．
2．用that不用which的七种情况：
①
先行词前有形容词最高级或
序数词修饰时，用 that不用which引导。如：
This is the best place that I have ever visited．
The second question that is asked is why we don't fall off the earth．
②先行词为little，much，all，
none，
few，（the）one， something，everything，nothing，anything等
不定代词时，用that引导。如：
There is nothing in the world that can frighten him．
③先行词既有指人的名词又有
指物的名词时，用 that引导。如：
He told us about the people and the cities that he had visited．
④先行词前有any，all，no，few，
every，some，little， much，（the）very，（the）only，（the）last
等词修饰时，用 that引导。如：
This is the very coat that I need．
Is oxygen the only gas that helps fire burn.
⑤
先行词为which时，为了避免重复，用that引导。如：
Which is the book that you bought yesterday.
Shanghai is no longer the city that it used to be in the 1930's．
⑦当主句是There be．．．句型时，用that引导。
There are two tickets for the film that are for you two．
结构
在“one of+
复数名词+
定语从句”结构中，从句的
谓语动词应与复数名词保持一致；当one前有the only修饰时，从句的谓语动词应与the only one保持一致。如：
He was one of the students who were late for class．
He was the only one of the students who was late for class．
引导词前有介词或短语介词时，修饰人只能用 whom，修饰物只能用which引导，如：
My glasses，without which I was like a blind man， fell to the ground and broke．
The man，by whom the wolf was shot，is called Jackson．