有时候逗号后是in which 有时是in what,但如果前面是介词只能用what么?

你对从句没有理解透.
in which常在定语从句中出现.
如: The house in which we had lived for ages was destroyed
in the flood.
此句中的in which 就是定从. 修饰 the house. in which=where
in what 一般会在名词性从句中出现.
如: I am quite interested in what you want for your birthday.
what you want for your birthday为in的宾语.