They both have Super Junior members. I don't think I have to say anything else.

Both about big sisters trying to keep their families from getting torn apart. Both fun and heartbreaking.

Both are about teens dealing with heart diseases and both have romance in it.

Both are about young people dealing with life threatening diseases and both have romance in it.

They're two versions of the same story (you can think of "Classmates" as a touching side story). In "Classmates" Jun meets a girl through e-mail and falls in love with her; in "Taiikukan Baby" Jun is courted by his rival and classmate (male).

They're both a collection of stories that are both fantastical and chilling. Neither are exactly horror, but both are horrifying. They're great for a lover of psychological horror. The stories are connected in Confessions, but that's a minor difference. The feel and setting of both films is similar.

The Oh Boy! project drama starts from the same main plot as the movie. We can say that it has its roots in the movie.

Although their genres are different, the atmosphere is similiar in both movies. Each movie has unearthly elements: in Postman to Heaven the protagonist is an angel, in Sweet Rain the main character is a grim reaper.

same director, also movie about little kids.