I've had more than my fair share of roleplays and fanfictions. and I've also had my share of seeing people use Korean incorrectly. Here is the compilation of common mistakes people make when using Korean. This list is going to be updated as I go so check in regularly!
01| Unnie? Oppa? Huh?
In South Korea, the cultural issue of respecting someone older than you is very important. This cultural phenomenon is the reason why Koreans use honorifics and other such qualities that are so thoroughly embedded into the everyday life of Koreans.
The terms oppa, unni, hyung and noona is a very normal, expected way to call older people. It shows respect and endearment.
If you're a girl, you say--- Unni to an older girl, Oppa to an older guy
If you're a guy, you say--- Noona to an older girl, Hyung to an older guy
Both girls and guys say--- Dongsaeng to girls and guys younger than them. Note (This is important! Because a lot of you make this mistake): The term dongsaeng, isn't usually used to call someone directly. it's more of a referring term. So while it's appropriate to refer to someone as your dongsaeng, you don't usually say "Hey what's up dongsaeng?"
It's also not appropriate to call someone who's a lot older than you with these terms. These are used only for close age gapped people!
So those of you who have Minho and Key calling each other hyung in your fanfic, go fix it now :p
02| Korean name suffixes
There are other honorifics and suffixes that follows a person's name when you call them. It's similar to the whole idea of using 'Mr. Mrs. or Ms.'
-nim: Formal; to someone with a high status; ex) 선생님 seonsaengnim meaning teacher
-sshi: Formal; used for both females and males of approximately equal status
-goon: Formal but less common; used for males
-yang: Formal but less common; used for females
03| Jonghyun-ah? Jonghyun-yah?
First of all, yes, Shinee is my bias group. So I will be using examples with their names.
You may have noticed people calling the other's name with either a '-ah' or '-yah' after the name. This is a similar concept to 02 but much informal. The endings are just naturally added after the name when you're calling them. Note, don't use this to call an older person because that would be very rude. You shouldn't be calling the name of someone older than you anyways.
-ah: Used when the name have a final consonant. (Remember? The lighter orange box for final consonants?)
ex) Taemin-ah because min; Kibum-ah because bum
-yah: Used when the name does not have a final consonant
ex) Minho-yah because ho (no final consonant); Key-yah because ey