-you call him "bác" if he's older than your father/mother (ba/má) and "chú" if ( ex: bác đi đâu vậy? where are you going?)
-you call her " bác/cô" if she's older than your ba/má and inverstly and vice versa, you call "cô"
( cô đang làm gì vậy? what are you doing?)
-someone who is the same age with you, or more or less a little---> call "bạn" (or " cậu" is ok)
-the girl that's older than you, ex: you're 20 and she's 21,25,30---> call "chị"
-the boy that's older than you-----------------------------------> call " anh"
-the boy or girl that's younger than you -------------------------> call "em"
- your father/mother-----> you call "ba/má" it's like you say dad/mom in english
-your grandfather/mother-----> "ông/bà"
*And remember, when you say, all these words are the same as "you" in english
Ok, some example:
+ ba đi đâu vậy?/ má đi đâu vậy?/bạn đi đâu vậy/chị đi đâu vậy... = where are you going?
+ ba/má/anh/chị/em/ông/bà/bạn đang ăn gì vậy? = what are you eating?
++ my father/my momther/my friend= ba của tôi/má của tôi/bạn của tôi
ba của tôi thì rất đẹp trai = my father is very handsome :D