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Both dramas have well chosen cast, focus on historical side of story, more accurately, both stories are about fighting agains Japanese. Authentic mise-en-scene, stunning cinematography.

Sungkyunkwan's scholars help prince Sado's son to clear his father's name and find lost important document. It's totally different story, but you can learn another version of prince Sado's tragic story.
If you liked prince Sado as a history character, you should watch this drama. (from this drama I first heard about prince Sado and since then he is my obsession)

Both movies have similar atmosphere from old times, so you get to enjoy old korean music, see old clothes styles and, also, imagine how korean teenagers lived when they didn't have mobile phones! Both movies are about friendship, hardships and, of course, first love. But I could say that "Hot Young Bloods" isn't as sad as "Unforgettable", and, in my mind, has more 'dirty' languages :) And if you like Korean satoori (dialects) you should watch both movies and fall for main characters cute satoori.
Did I mention that actors are great in both movies? Yes, they are. "Unforgettable" has Kim Soo Hyun (The moon that embraces the sun, I miss you, School who are you) and D.O (It's okay that's love). And "Hot young bloods" has Park Bo Young (Werewolf boy, Oh my chost) and Lee Jong Suk (I hear your voice, Pinocchio, Doctor stranger, Secret Garden, School 2013) as main leads.

"Unforgettable" is set in 90s but shows how main characters are living in modern world. Like "Reply 1988", it's all about childhood friends which lived in a small village, and their first love and dreams.
Also, "Unforgettable" actors are really good. I could say as good as "Reply 1988". And, of course, in both you could hear old times music. Don't know about you, but I really love listening to korean music from 70s to 90s.

-Poor Female Leads
-Rich Male Leads in a position of power
-Contract Romance and Marriage
-Cohabitation
-Male Lead has girlfriend in the beginning that the Female Lead "ruins" by events out of anyones control
-Sweet Second Male Lead
-The Second Female Lead and First Male Lead try to continue a relationship before guilt for the Female Lead gets in the way for the Male Lead
-Annoying Time Jump

Both dramas presents story about a girl and a guy who share the same room and later fall in love with each other.

Eat Drink Man Woman is an older Taiwanese movie and Let's Eat is a Korean Drama, but they both have romance and Food.

They are both about a romance between the king, and a lowly peasant who later becomes one of his concubines. There is also hate towards the main lead girl due to her low social status, and if you have seen Dong Yi, Yi San is King Sukjon's (the king in Dong Yi) which is also the king in Jang Ok Jung, is his great-grandson.

Very very similar, in terms of main cast, costume, plot, even setting, etc.
let's just say its a new Shomuni (5). While Shomuni 4 is a sequel of the past dramas (similar title), Age Harrasment is a another alternate ver. of the franchise.

Aaron plays pretty much the same role (CEO of a company) and the female lead also works for the company.

Both dramas are stories of revenge. The series surrounds the fulfillment of revenge for a beloved person who died violently when the main characters were younger. The stories' conclusions are reminiscent of each other. Ikuta Toma also stars in both dramas.

The story line is almost the same... The main male lead in Ex files 2 kinda reminds me of Rain.

Beautiful Gong Shim seems to be comedy/romance with some drama and Can You hear my Heart is drama/romance with some comedy. While Drama/romance with comedy is quite different from Comedy Romance with some drama, but I think you will find both dramas romantic storyline satisfying.
What they have in common is the actor Nam Goong Min playing a lead role that is wrapped in mystery pertaining to his childhood/parents/identity.

Both are romance dramas which presents love story between human and robot/cyborg.