반말 Vs 존댓말
A. 존댓말 has respects, manners in the words so we use these words to older people, strangers, group of people (no matter with age this case), and colleagues
most of case of 존댓말 has 요,ㅂ니다 that end of 반말
ex: 반말: 반가워 , 존댓말: 반가워요 / 반말: 고마워, 존댓말:고마워요
B. 반말 is used among close people as family, friends, or to young people(not stranger).
First of all we use 존댓말 till we feel close. It's no matter with age but if it has too big age gap, I recommend to use 존댓말.
반말 makes people feel close so after became friends we use 반말 as always.
So learn 반말 first and after learn how to make 존댓말 (반말+요)
That's easy. isn't it?