The dreamy music at the beginning and a little bit further on is the famous Canon in D written by Johann Pachelbel around 1680.
The pop music that is played after the class when the soldiers run out is Doo-wah-diddy, originally by the Exciters but made famous by Manfred Mann in 1964. Later, at the folk dance class Hippy hippy shake performed by the Swinging Blue Jeans and brought out in 1963 is played.
There are many parallels between the flashback and present-day stories. In the flashbacks, Joon Ha writes letters in the name of his friend Tae-soo to their mutual love interest. In the present day, Ji-hye writes letters in the name of her friend to their mutual love interest. In the flashbacks, Joo-hee has an annoying female friend that Joon-ha is initially set up with. In the present day, Ji-hye also has an annoying friend that interferes with her relationship. Also, Joo-hee and Ji-hye both receive the same poem from the people they love: "When the sun shines...".