Recommendations

Although these dramas aren't at all alike in storyline or theme, they're both fluffy, fun and light dramas that are feel-good romantic comedies. Both have very lovable male leads who fall head-over-heels for the female lead, both of whom are a bit ditzy, scatterbrained and ignorant in the field of what life has in store for them. Both dramas circle around the marriage theme and the characters' future. Both of these dramas are on my top 10 list and are very worth watching when you're in the mood for something super romancy and heart-pounding.

Both are revenge-based dramas in which the male lead is wrongly charged with murder and returns several years later with more power and tries to ruin his accusers. Both have strong female leads who are determined and can handle themselves without the help of a man. Both these dramas are melos and have a really sweet feeling and can get you sucked into the life of the characters immediately and of course.. both are very enjoyable. Although I must say that I prefered Beyond the Clouds more than I liked Goodbye Mr. Black.

While this movie is about a husband talking to his dead wife, Signal is a drama about a young detective using an old walkie talkie to communicate with a detective from the past and solve cold case murders.

Both take a place in Shangai during the 1930's. And the leading couple are similar: the rich and dominate guy with sweet and naive girl. Cruel romance is just more agresive, when the girl wearing tassel is more sweet.

Both dramas presents story about a girl who lives with some guys in one house and later she falls in love with one of them. But plot and atmosphere in these dramas are different.

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.