关系代词和关系副词的作用：
先行词指物时， 关系代词用that 或which(在句中作主语或宾语)。
如：China is a country which / that has a large population.
The computer game which/ that he is playing is his favorite.
先行词指人时， 关系代词用(who)作主语或宾语，whom (作宾语)，whose (作定语)，that (作主语或宾语)。 如：
The man who is mending the machine has been retired.
This is the policeman who/ whom they were talking about yesterday.
I like the girl whose mother is an actor.
注意：下列情况下关系代词只用that 而不用which 或 who。
先行词是指物的不代词，如：all, little, few, much, none, anything, nothing, everything等时。如：I didn’t understand the words all that he said.
先行词由最高级、序数词以及any, very, only, all, no, just 等所修饰。如：This is the most beautiful place that I have been to .
行词既包括人，也包括物。如：They always like to talk about the man and the things that they see on the road.
当主句是以which 或who 开头的特殊疑问句时。如：who is the man that opened up the lab.