which 与 that 在定语从句中 被称为：关系代词
which 引导定语从句时,先行词是物,在从句中可做主语、宾语、表语
that 引导定语从句时,先行词既可以是人,也可以是物,在从句中也可做主语、宾语、表语
关于which和that的常考点：
一、在限制性定语从句中,只能用that不能用which的情况：
1）当先行词是指物的不定代词
例：Do you have anything that you want to say for yourself?
2）当先行词前面有下列词修饰时：the only,the very,just,any,every,some等
例：This is the very busy that I’m waiting for.
3）当先行词是形容词的最高级或先行词的前面有形容词最高级修饰时
例子：This is the best that has been used against pollution.
4）当先行词是序数词或前面有序数词修饰时
例：The train is the last that will go to Suzhou.
5）当先行词既有人,又有物
例：Do you know the things and persons that they are talking about?
6）当主句的主语是疑问词 who 或 which 时
例：which is the bike that you lost?Who is the boy that won the gold medal?
7）主句是there be句型且关系词在从句中作主语时
例：there is a seat in the corner that is still available.
二、当先行词指物,定语从句中只能用which不用that的情况：
1）在非限制性定语从句中,只能用 which
2）当动词短语中的介词提前时,只能用 which
3）先行词后面有插入语时,只能用which.
例：Here is the English grammar book which,as I’ve told you,will help you improve your English.
4）先行词本身就是that时
例：What’s that which flashed in the sky just now?