a. When you ask something to a older person than you (most of case to children, but actually people use it to ask something a little more lovely) you use ~니 at the end of the question
1. 먹었니? did you eat? (lovely)
2. 오늘은 헬스장에 갔니? did you went to the gym? (lovely)
3 오빠, 이걸 날위해 샀니?oppa, did you buy this for me? (lovely)
b. And when you want yo say (of course ....) or justo want yo add (_______, right?) you use the end 지 or 죠 (formal)
1. 오늘은 날씨가 넘 좋았지? today the weather was good, right?
2. 걔가 진짜 이쁘지? that girl is pretty, right?
3. 오늘은 너와 함께 영화권에 가지. of course today I will go with you to the cinema.
4. 여자친구 없지. of course I don't have girlfriend.