1、在非正式文体中，用于指人的关系代词who whom, that 指代先行词在定语从句中作宾语时通常可以省略；用于指物的关系代词which和that指代先行词在定语从句中作宾语时中也通常省略。例如：

    1）The man (whom/who/that) we have just seen is a famous writer. 

    2）Where is the book (which/that) I bought last week? 

2、 关系代词指代先行词在定语从句中作表语时，只用that ，它既可以指人，也可以指物，但时常省略。例如： 

     1）He is no longer the man that he used to be. 

     2）This is no longer the dirty place (that) it used to be. 

3、以the way为先行词的限制性定语从句，如果先行词way在定语从句中做状语，通常以in which或 that 引导，但在非正式文体中，人们通常省略in which或 that 。例如;

          1）The way (in which) he spoke to us was suspicious. 

          2）I don’t like the way (that) you laugh at her. 

4、以place为先行词的限制性定语从句，如果先行词place在定语从句中做状语，通常以where 引导，但在非正式文体中，人们通常省略where。例如：

      1）This is the place (where) we met yesterday. 

5、以reason为先行词的限制性定语从句，如果先行词reason在定语从句中做状语，通常以why引导，但在非正式文体中，人们通常省略why。例如：

       1）That is the real reason(why) he did it. 

6、以time,day,week,month,year等为先行词的限制性定语从句，如果先行词在定语从句中做状语，通常以when引导，但在在某些句型和某些时间状语中，人们通常省略when。例如：

       1）But help never stopped coming from the day she fell ill. 

       2）Each time he came, he did his best to help us.

注：针对以上情况，下列情形定语从句的引导词不能省。

       1、如果作介词宾语且介词提前，这时关系代词指人用whom,指物用which，而且均不能省。例如：

         1）Gun control is a subject  about which Americans have argued for a long time.

         2）  She brought with her three friends,none of whom I had ever met before.

2、在非限制性定语从句中，无论是关系代词还是关系副词均不能省。例如：

         1）They have won their last three matches, which I find a bit surprising actually

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