안녕 (Annyeong) - It does has alot of meaning depending on how it is phrase..
1) Hi! / Hello
2) How are you?
3) Good Morning/ Good afternoon/ Good evening/ Good day! (Basically a form of greeting)
*Annyeong (adult saying to kids/young person or how we greet our besties/ friends)
*Annyeong Hasseyo (same age/level of profession - Formal & polite way)
*Annyeong Hashimnika (young person to higher authority as a sign of respect say this to the parents, old folks, employer, teacher, Royal family, police, person in higher rank)
But Annyeong basically has TWO different meaning:- Hi & Bye!
It can mean Hi!
*Annyeong unni, you look pretty today! ^^*
It can also mean Bye!
*Annyeong, Hope to see you soon in Korea!*
So, how do we differentiate all these apart??! Hehehe...simply..practice makes perfect! You have to practice either by watching K-dramas, get yourselves a korean pen-pal, have a conversation with a korean, etc... ^^
Happy practising guys!