Hi 세스!!
Let me help you so that your brain won't explode.

이것은 제가
산 책입니다.
-> This is a book I bought. (talking to your friend at a book store)

이것은 제가
사는 책입니다.
--> This is a book I am buying. (someone touched a book you chose at a book store.)

이것은 제가
살 책입니다.
--> This is a book I will buy. (talking to your friend at a book store )
Did your brain want to grasp this? No??
화이팅 세스 님!