너 시끄러워.(very informal) 당신 시끄러워요.

you use '돼' often when you are not so sure or have no idea about what you are asking. sounds like really relying on the other's opinion. ex) 먹어도 돼요? 선생님! 이제 집에 가도 돼요? 레코드샵 가려면, 이 쪽으로 가야 돼요?

as you mentioned its exactly should have + done 1) 공부를 더 열심히 할 걸(=것을)/ 그랬어. studied hard / i should have 2) 너 만나기 전에/밥 먹지 말 걸(=것을)/ 그랬어. eaten before i meet you/ i should not have in speaking we often skip '그랬어' part. and drag '걸' sound longer. i dont add 그랬어 much. ex) 공부 좀 더 할 걸... 돈 좀 더 가져 올 걸... (I should've brought more money) or it can be also 그녀랑 더 많은 시간을 보낼 걸 그랬어 i wish i would spend more time with her

보다(in this case =will check on) + ㄹ(to make verb adjective and future tense) + 일(work)+보다(to check)+하러 or 러(to do) +요(ending). so it is like to check on something i will(need to) check on

1) you could use it when you want to remind of something someone has forgotten. maybe when you want to complain. ;) a: 빨리 나가자 (let's leave here quickly) b: 야! 나 아직 먹고 있잖아(hey! im still eating) 2) you could also use it when you want to remind someone of good things. a: 술 먹으러 가자 (let's go for drinks) b: 안돼(no way) 너 내가 평일에 술 안 먹는거 알잖아(you know i dont drink during weekdays) c: 바보(babo). 오늘은 금요일이잖아(today is friday). we dont work tomorrow(우리 내일 일 안해)

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