定语从句in which 的用法

掌握介词+关系词引导的定语从句的关键：
1. 明白关键代词的作用，关系代词在定语从句有三个作用：
(1) 引导定语从句
(2)替代先词
(3)充当定语从句。
2. 关系代词作介词的宾语，一起构成介宾短语引导定语从句，因此，只能用which指物，用whom指人。
3. 介词的两个来源：
(1) 和前面先行词形成搭配
(2)和后面动词形成搭配。

你的问题中提到的in which是属于“介词+关系代词”型的定语从句.
关系代词在定语从句中做介词宾语时,从句常由介词 + 关系代词（只用 whom /which）引导
The school (that/which) he once studied in is very famous.
= The school in which he once studied is very famous.
Tomorrow I will bring here a magazine (that/which) you asked for.
= Tomorrow I will bring here a magazine for which you asked.
We'll go to hear the famous singer (whom/that/who) we have often talked about.
= We'll go to hear the famous singer about whom we have often talked.