一、你提到的问题比较笼统,猜想你是想知道什么情况下 in which 或 for which 可以用 that 代替吧.
如果关系代词指代的是表示方式的先行词（如way、manner、measure 等）,那么 in which 既可以用 that 也可以用 how 取代,还可以省略不用.如：
He wants to have his own way (in which) he can do whatever he likes.（希望有自己为所欲为的方式.）
He wants to have his own way (that) he can do whatever he likes.
He wants to have his own way (how) he can do whatever he likes.
He wants to have his own way he can do whatever he likes.
This is the best measure (in which) you can find out the thief.（这就是你抓住那个贼的最好手段.）
This is the best measure (that) you can find out the thief.
This is the best measure (how) you can find out the thief.
This is the best measure you can find out the thief.
同理.当关系代词指代的是表示原因的先行词（如 cause、reason、matter 等）时,in which 既可以用 that 也可以用 why 取代,当然也可以说省略不用.如：
The cause (in which) we support them is unquestionable.（我们援助他们的理由是不容置疑的）.
The cause (that) we support them is unquestionable.
The cause (why) we support them is unquestionable.
The cause we support them is unquestionable.
二、“介词 + 关系代词” 关系代词是 which,指代的是事物,关系代词是 whom（不能用主格 who） 指代的是人.
三、“介词 + 关系代词” 也可以把介词后置,如：
This is the house (in which) I used to live./This is the house (which) I used to live (in).这是我曾经住过的房子
The man (whith whom)I once worked is one of my friends./ The man (whom/who) I once worked (with) is one of my friends.我曾经共事的那个人是我的一个朋友.【介词后置,可以用主格 who 代替 whom】