All the chapters that have "for you" are present.
"The past" explains what happened between them that lead up to this.
The whole story is very vague, but on purpose.
The little notes at the end that are bolded in this font, mean that it was a piece of the present time.
Joon was the one who narrated it all.
Some of you may have noticed this sentence from "the past; 3³" that it says, "our love was a spark. Nothing special. Yet you realized it before I did."
That is Namjoon's POV. Namjoon never once saw their friendship that way. Jin was the one who realized that there was something. Yet there was nothing.
Jin slowly gets jealous but is too nice to show it. He'll do anything for joonie. Even death.
In the second to last chapter Jin asked for a hug. That hug was Jin accepting the cruel reality that he can't have Joonie.
"Bye bye baby now" comes from the song "six feet under" by Vixx. I used the lyrics to symbolize Jin's last words.
Vixx's concept for Zelo was envy and jealousy which is what consumed Jin to the point he wanted to kill Namjoon.
In the last chapter it shows that Jin is dead and Namjoon is in the mental hospital.
It says that he loved him. That means after Jin died he realized his true feelings for him. Namjoon eventually went insane because he killed the person he loves.
He swears he did .
Hopefully that helps ^°^