什么时候用that,which,who,whom?不懂!

who whom都用于人的 但前者范围更广 。后者是宾格，前者是主格。that which都用于物 但后者范围广 that后面不能有逗号，前面不能有介词 ，有anything,nothing,evrything……时用that。当which能充当后面动词的宾语时 就能用

大多情况下两个可以混用
先行词和定语从句之间有逗号隔开或有介词链接时，只能用which
先行词是anything,nothing,something，everything...时，只能用that引导定语从句
例：They played computer game in night,which made the teacher angry.
There are so many houses,one of which is my sister's.

who和whom是修饰人的
who是主格，whom是宾格
看先行词在句中的成分来决定用who还是whom
一般情况下，能使用whom时都可以用who代替，或whom省去，因为whom一般情况下是修饰宾语先行词
有介词连接的情况，只能用whom
例如：he is the boy who won in the match.
he is the boy who/whom(或省去) teachers do not like.
they are students, most of whom are good at English.

"名师指导：英语从句总复习-中考英语"
(不好意思,因为内容太多了,我就不复制上来了哈,网址我已经贴上来了,讲得还蛮详细的,..)