-(으)면 안 되다

States that an action is not allowed or not acceptable: 'you must not… / it's not okay to…'.

능력·허락·의무Can, may & must

Han: The fixed expression uses 되다 ('be acceptable'), so it is 안 돼요; 안 해요 just means 'I don't do it'.

Examples

이 약을 먹고 술을 마시면 안 돼요.You shouldn't drink alcohol after taking this medicine.

좀 더운데 창문 좀 열면 안 돼요?It's a bit hot — could I open the window?

수업 시간에 휴대폰을 쓰면 안 됩니다.You must not use your phone during class.

여기에서 사진을 찍으면 안 돼요.You must not take photos here.

-(으)면 안 되다vs-아/어도 되다-아/어도 되다 gives permission (the opposite meaning): 여기 앉아도 돼요? — 네, 앉아도 돼요 / 아니요, 앉으면 안 돼요.

V/A+(으)면 안 되다

먹다 → 먹으면 안 돼요, 가다 → 가면 안 돼요

After a final consonant use -으면 (먹으면); after a vowel or ㄹ use -면 (가면, 만들면 — not 만들으면).
As a question, -(으)면 안 돼요? politely asks for permission, like 'Could I…?': 창문 좀 열면 안 돼요?
It also works with adjectives to say what something must not be: 너무 비싸면 안 돼요.
The negation goes on 되다: say 안 돼요 / 안 됩니다, not 안 해요 or 못 돼요. For 'must do', use -아/어야 되다 instead.

Wrong: ~~여기에서 담배를 피우면 안 해요.~~

Right: 여기에서 담배를 피우면 안 돼요.

The fixed expression uses 되다 ('be acceptable'), so it is 안 돼요; 안 해요 just means 'I don't do it'.

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