(으)면 안 되다 (must not, not allowed)

Let's look at the logic behind the words. It is actually made of two parts. First, we have -(으)면. This means "if." Second, we have 안 되다. This means "it does not work" or "it is not okay." So, when you put them together, you are literally saying "If you do [verb], it doesn't work." It is a very logical way to forbid something. If you eat this, it's not okay. If you go there, it doesn't work. It sounds a bit dramatic in English. But in Korean, it is perfectly natural. You are setting a condition that leads to a bad result. It is like saying "This path is blocked." It is much stronger than just saying "Don't." It implies there is a rule or a reason. Even native speakers use this to talk to themselves. Have you ever looked at a delicious cake while on a diet? You might whisper, 먹으면 안 돼. "I shouldn't eat it."

1. 1Building this grammar is like playing with Lego blocks. You just need the verb stem and the right ending. Follow these simple steps:
2. 2Find the verb stem by dropping 다 from the dictionary form.
3. 3Look at the last letter of the stem.
4. 4If the stem ends in a vowel, add -면 안 되다. For example, 가다 becomes 가면 안 돼요.
5. 5If the stem ends in a consonant, add -으면 안 되다. For example, 먹다 becomes 먹으면 안 돼요.
6. 6Here is the tricky part! If the stem ends in ㄹ, treat it like a vowel. Just add -면 안 되다. So, 열다 becomes 열면 안 돼요. Do not say 열으면! That is a common trap for beginners.
7. 7For the ㅂ irregular verbs like 줍다, it changes to 우. So it becomes 주우면 안 돼요.
8. 8Remember to conjugate 되다 at the end. Usually, we use 되다 -> 돼요 (polite) or 돼 (informal).

Use this when there is a clear rule. This could be a law, like 여기서 담배를 피우면 안 돼요 (No smoking). Use it for social etiquette too. In Korea, you shouldn't wear shoes inside a house. You would say, 신발을 신고 들어가면 안 돼요. It is also great for health or safety advice. If your friend is sick, tell them 너무 많이 일하면 안 돼요. You are saying "Working too much is not good for you." You will hear this a lot in professional settings. If you are in a job interview, don't use slang! 속어를 쓰면 안 돼요. It shows you understand the boundaries of the situation. It is also used in games or sports. "You can't touch the ball with your hands!" 손으로 공을 만지면 안 돼요. It is the language of "No-Nos."

Do not use this for polite requests. If you want someone to please not do something, use -(으)시지 마세요 instead. (으)면 안 되다 can sound a bit like a lecture. Use it sparingly with people older than you. Unless you are a doctor or a policeman, it might sound rude. For example, don't tell your boss 지각하면 안 돼요 (You shouldn't be late). That is a bit too bold! They might give you a funny look. Also, don't use it for things that are just impossible. Use the "cannot" grammar -(으)ㄹ 수 없다 for that. This grammar is specifically about permission and prohibition. It is not about ability. If you say "I can't fly," don't use 면 안 돼요. Use it only when someone has the choice to do the wrong thing.

The biggest mistake is the spelling of 돼요. Many people write 되요. That is wrong! Think of it as 되어 + 요 = 돼요. Another mistake is forgetting the ㄹ rule. People often say 만들으면 안 돼요. Please don't do that! It should be 만들면 안 돼요. It sounds much smoother. Also, watch out for the ㄷ irregulars. 듣다 becomes 들으면 안 돼요. If you say 듣으면, people will still understand you. But they will know you are a student! Another funny mistake is using it with nouns. You cannot just add it to a noun. You must use the verb 이다 (to be). So, "You shouldn't be a liar" is 거짓말쟁이면 안 돼요. Finally, don't forget the 안. If you leave it out, you are saying the opposite! 먹으면 돼요 means "It's okay to eat."

You might know -지 마세요. What is the difference? -지 마세요 is a direct command. It is like saying "Stop!" or "Don't do that right now." (으)면 안 되다 is more like a general rule or a statement. If a child is running, you say 뛰지 마세요! (Stop running!). If you are explaining the rules of a museum, you say 여기서 뛰면 안 돼요 (Running is not allowed here). See the difference? One is a command, the other is a rule. We also have -(으)ㄹ 수 없다. This means "I am unable to do it." For example, 매운 것을 먹을 수 없어요 means "I can't eat spicy food (my stomach hurts)." But 매운 것을 먹으면 안 돼요 means "I'm not allowed to eat spicy food (the doctor said so)."