안녕히 가세요. 안녕히 계세요. You and A are here. 1) A leaves and you stay. 안녕히 가세요. 가 is from '가다=go' 2) Both A and you leave 안녕히 가세요. 3) A stay and you leave. 안녕히 계세요. 4) On the phone or in a letter. If choose one of them, it'd be 안녕히 계세요. (잘 있어) Assume you meet someone on the line or in a letter and you leave first. But it can be anything such as 'thank you. 고마워요.(고마워)' ' See you tomorrow 내일 봐요(내일 봐).' 잘 가 = 안녕히 가세요(more formal). '안녕' can be used as 'hi' or 'bye' between friends. I'd like to tell you that there're basically two level of formality of Korean language. One is 반말, which is used to friends who's younger than you or the same age. The other is 존댓말, which is used to order person and anyone you don't know well. (Such as, someone you meet for the first time, someone who hasn't yet told you, "you can use 반말 to me.") 존댓말 normally end with "-요. ~요?, ~니다. ~니까?" 안녕히 가세요 (계세요), 내일 봐요, 고마워요, 저는 미희예요. 저는 미희입니다. - 존댓말 잘 가 (있어), 안녕, 내일 봐, 고마워, 나는 미희야. 나는 미희하고 해. - 반말

Formally: When you're leaving - 안녕히 계세요 ("Stay peacefully") When people are leaving - 안녕히 가세요 (The way I remember how to distinguish these is 가다 means "to go" so you're telling them to "go peacefully") Informally: 안녕 for both cases.

i just say 안녕 ^^