Explanations of Scenes

WARNING SPOILER AHEAD

The two characters that tied the story between past & present:
1. Ye’s father: He was Lu’s teacher 20 years ago, and took a photo with Lu on the fateful day Lu supposedly passed away from an asthma attack. When Ye visited Lu’s home 20 years later, he saw his father posing with Lu 20 years ago, returned home and asked father about it and realized what happened.
2. York (大勇): School janitor. In the past, he was younger and was normal. In the future, he seemed to be retarded due to an accident.
“Secrets” & the Grand Piano in old piano room
It turned out that the key to crossing time was by playing the piano piece “Secrets” on the old grand piano in the old piano room.
That was why Lu told Ye never to play “Secret” in the old music room, which Ye thought she doesn’t want him to spend time with Sky.

“Secrets”
“Follow the notes upon a journey
At first sight marks one's destiny
Once the voyage comes to an end
Return lies within hasty key”
Interpretation:
Playing the notes will allow one to embark on a journey The first person you see will be the one who will be able to see you To return to the present, play the piece quickly
“At first sight makes one’s destiny” After the first time Lu played “Secret”, Ye was the first person she set sight on, and that her destiny was tied with him.
That also explains the following scene:
1. Why Lu closes her eyes and counted her steps from the old piano room to Ye’s classroom, as well as the scene when after she took her last photo with her teacher, she closed her eyes, slowly climbed the steps leading to the school hall and only opened her eyes when she was outside the school hall.
2. The moment she opened her eyes, she saw Qin Yi. Subsequently during the piano battle, Qin Yi and Ye exchanged words. However Ye did not see her in the crowd.
3. There was a scene when York (Da Yong) and Lu hurried covered their eyes when Lu stepped out of the old piano room. York commented that he thought Lu was trying to play her way back (into the past) and apologized for foiling her attempt to look for Ye.

Le Cygne The Swan
Ye dueled with his classmate Howe in a piano battle over a manuscript Le Cygne, which he subsequently gave to Lu.
This was the piece played by the orchestra during the Ye’s graduation ceremony, Ye’s present to Lu.
When Ye was in Lu’s room, he saw the manuscript Le Cygne that he gave to Lu. Lu’s mother quickly snatched the manuscript from Ye, and found a sketching that Lu did years ago. The sketching was of Ye.
Le Cygne, or The Swan, is the thirteenth movement of The Carnival of the Animals by Camille Saint-Saëns. This piece features a solo cello in tenor clef and an accompaniment piano. This piece is not to be confused with the famous ballet Swan Lake.

《情人的眼淚》" Lovers Tears"
Yao Su Jung's old song "Lovers’ Tears" is the song that Ye likes. After school, Ye brought Lu to a CD shop. Ye put the headphone on Lu to let her hear the song he liked, and Lu was struck. This was the song she liked.
20 years later, when Ye visited Lu’s home, this was the music being played, to which Lu’s mother commented that this was Lu and her boyfriend’s favorite song.

Source:Wiki