which引导定语从句
This is my house， which)is small and old. 这是我的房子，它又小又旧 is studying economics， (which subject) is very important. 他正在学习经济学， 该学科是很重要的.
比如:This is the place in which heused to stay.
in which 只能用在定语从句中，等于where，在定语从句中作状语in which可以翻译成在……里面
The school (that/which) he once studied in is very famous.
= The school in which he once studied is very famous.
谈到which和in which用法的区别，应该指的是它们在定语从句中做关系词，使用which还是in which，取决于将先行词代入，从句之后，它从句中所做的句子成分，如果他在从句中作主语，宾语或表语就用which，如果在从句中作地点状语和时间状语，就用In which，例如，she speaks of the day in which he got the place which he worked in is famous.
