It's all about first love. Both of them met in the young age and they started a beautiful story of first love. They were both also separated from each other because of unexpected factors in their life.

The difference = the adult version of 2521 didnt meet and it wrapped up as first love story without reunion. Hatsukoi got happy ending and both of the leads are ended up together.

Set in the early 12th century, war threatens to erupt across the land of Da Chu, and the Imperial court is filled with corruption and intrigue. The cruel Emperor and his corrupt cronies plot to kill the Empress Yao Mo Xin – and are successful in their malicious efforts. Meanwhile, other vindictive parties chase down Yao Mo Wan, the Empress’s younger sister. In the night full of mystery, the dead queen’s spirit enters the body of her younger sister, intent on taking revenge on people who wronged and poisoned her to death. The Empress in Yao Mo Wan's body, enters the Imperial Court, bent on seeking vengeance. She also meets (and falls for) Ye Jun Qing, a young Da Chu prince. As she discovers the scale of the corruption at court, she decides to do her best to see that Ye Jun Qing deposes the evil Emperor. She uses the best tools at her disposal: her wits and natural intelligence to help her beloved in getting the throne.

Both dramas involved divorce couple who come to reunited again to help their child from blood cancer. They need to create the second child to donate high rate matching bone marrow. But the problem is both of them are already married to other people.

Left Right Synopsis
"The birth of a child has caused an inflection point and a rift in the intimate relationship of a loving young couple, and the once affectionate campus couple eventually goes their separate ways. After the baptism of life, they move forward steadily in their respective life tracks. However, the child's sudden onset of leukemia disrupts the originally peaceful reunited family. The divorced couple's peaceful lives are forced to let go of their differences due to the child's malady and join hands again to fight against fate. The fate of several families is thus closely linked. In the process of saving the child, the adults are also being saved by the child, reconciling with the past and themselves."

Both Main Characters are unsatisfying with their current life. So when "something" can change their fate and life. They decided to change their current life into the new one.

The difference
- The Method of "How to Change the Fate"
- Winter Night takes place in "Another World". So, the FL replaced "Another Her" who got 180 degree life opposite hers in parallel world.
- The Golden Spoon takes place in the same world but the ML replaced someone else's life to change his fate

Best friends turned into lovers. But there is a rough process before that two become a couple and admit that they loved each others

The ML n FL were connected into particular time and they solved the case together. Their current versions were separated by one month apart.

Synopsis

Living a precarious life as a part-timer at a convenience store, Ae Ri strives to support her mother who is on a waiting list for a heart transplant. Unfortunately, her mother’s worsened physical condition leads her to forgo the long-awaited opportunity for the transplant, and just like that her mother disappears. Things were already chaotic enough for Ae Ri when she receives a phone call from a strange man claiming to be from the same world but 31 days in the future. Ironically, he desperately asks Ae Ri for help. Seo Jin, a father and a husband of a close-knit family, has been living a successful life as the youngest person to become a director at a construction company. One day, it all crumbles down with the kidnapping of his daughter and his wife’s suicide. On the verge of breaking down, he is given an opportunity to restore everything that was lost under one condition, help from a strange woman. To bring his daughter and wife back to life, he makes a phone call as the clock strikes 10:33.