請問by which 及 in which點用

首先明白最簡單的relative clause :
It is a book . I like the book.
連接後 : It is a book which I like.

之後到 Prep + which( relative pronoun ) 的用法 :

I find a house. I find a man in the house.
用relative pronoun 代了 house 來連接兩句的時候 ,正式用法會連 house 前的prep 都會放在relative pronoun 前面

Eg : I find a house in which I find a man.

The knife is sharp. I kill him by the knife.
連接後: The knife by which I kill him is sharp.

其實唔只by ,in ,其他prep 都有這用法