-(으)면 안 되다 expresses the prohibition or limitation of a particular action of the listener.

It also can represent social conventions or common sense dictating that a behavior or state is prohibited or not tolerated.

It corresponds to 'may not' or 'not allowed to' in English.

When the stem ends in a vowel or ㄹ, -면 안 되다 is used, and when the stem ends in a consonant, -으면 안 되다 is used.

Details

-(으)면 안 되다 can be used in the double negative form, that is, -지 않으면 안 되다 to emphasize that a particular behavior must be performed.

8월은 휴가철이니까 비행기 표를 미리 사지 않으면 안 돼요. (= 표를 미리 사야 돼요.)

August is vacation season, so (you) must buy plane tickets in advance. (Plane tickets have to be bought in advance.)

병이 심각해서 수술하지 않으면 안 돼요. (= 수술해야 돼요.)

Because the illness is serious, surgery must be performed. (We have to perform surgery.)

다음 주에 중요한 시험이 있어서 공부하지 않으면 안 돼요. (= 공부해야 돼요)

Because (I) have an important test next week, I must study. (I have to study.)

Used to say a particular action is not allowed.

Details

Double Negative

It can also be used in the double negative form that is, 지 않으면 안 되다, to emphasize that a particular behaviour must be performed.

병이 심각해서 수술하지 않으면 안 돼요.

Because the illness is serious, surgery must be performed.

A/V-(으)면 안 되다