선행명사 전치사 + 명사 

시간 장소 원인 방법

When: in/at/on which

Where: in/at/on which

Why: for which

How: in which .that

I like Seoul. He was born in Seoul

I like Seoul, in which he was born. I like Seoul ,where he was born. 

Do you remember the day? They met for the first time on the day.

Do you remember the day on which =whenthey met for the first time?

The reason must be simple

I am seaching for a reason.

The reason for which I am searching must be simple.