-으면 안 되다

V/Adj/이다,아니다+ (으)면 안 되다 (-으면(if~) + 안 되다(it’s not okay)) This indicates prohibition or that something is not allowed. (You should not~, You’re not allowed to~)

길에 쓰레기를 버리면 안 돼요.

You shouldn't litter on the street.

여기에서 사진 찍으면 안 돼요?

Is it not allowed to take pictures here?

저는 알레르기 있어서 밀가루를 먹으면 안 돼요.

I shouldn't eat flour because I have allergies.