-아/어서는 안 되다 Grammar in Korean: Meaning and How to Use It

What you'll learn

• Attach -아/어서는 to any stem, choosing between -아서는, -어서는 and 해서는.
• Say "You must not..." in a firm, formal way.
• Read signs and rules that use 안 됩니다.
• Use the spoken contraction -아/어선.
• Tell -아/어서는 안 되다 apart from -(으)면 안 되다, -지 마세요 and -아/어도 되다.

Before you start: you should know how to find a verb stem (drop 다 from the dictionary form) and the everyday prohibition -(으)면 안 되다. Audio plays only when you press a button.

What Does -아/어서는 안 되다 Grammar Mean?

-아/어서는 안 되다 grammar means "you must never do it that way" or "it is wrong to do this." It is built from three parts. The first is -아/어서, a connector that links an action to what follows. The second is 는, the topic particle, which singles out the action as the subject of the rule. The third is 안 되다, which means "is not acceptable." Together they say "as for doing X, it is not acceptable."

Here are the first two examples. The pattern is the same in both, and only the verb changes.

• 여기에 쓰레기를 버려서는 안 됩니다. (yeogie sseuregireul beoryeoseoneun an doemnida.) [여기에 쓰레기를 버려서는 안 됨니다] = "You must not throw trash here."
• 약속을 어겨서는 안 돼요. (yaksogeul eogyeoseoneun an dwaeyo.) [약쏘글 어겨서는 안 돼요] = "You must not break a promise."

A 하다 verb gives another principle: 거짓말을 해서는 안 돼요 (geojinmareul haeseoneun an dwaeyo) [거진마를 해서는 안 돼요] = "You must not lie."

The first sentence is a sign. The second is a principle. Both state a rule that applies to everyone, not a warning to one person. That general feeling is the key difference from the everyday prohibition. The next section shows how to build the pattern.

How Do You Form -아/어서는 안 되다?

-아/어서는 안 되다 is formed by dropping 다 from the dictionary form, adding -아서는 or -어서는 to the stem, and then adding 안 되다. The choice between 아 and 어 follows vowel harmony. If the last vowel of the stem is ㅏ or ㅗ, add -아서는. For every other vowel, add -어서는. For 하다 verbs, the result is 해서는.

The table shows one rule applied to seven verbs. Notice that the form before 서는 is the same as the polite verb ending without 요: 버려요 becomes 버려서는. If you know the polite present, you can build -아/어서는 by removing 요 and adding 서는. The next three parts look at the groups of stems one at a time.

Stems Ending in a Vowel

A vowel stem often shortens when 아/어 is added. A stem ending in ㅣ joins 어 into ㅕ, so 버리 + 어 becomes 버려. A stem ending in ㅜ joins 어 into ㅝ, so 피우 + 어 becomes 피워. A stem ending in ㅐ keeps one vowel, so 내 + 어 becomes 내. Examples:

• 음식을 남겨서는 안 돼요. (eumsigeul namgyeoseoneun an dwaeyo.) [음시글 남겨서는 안 돼요] = "You must not leave food."
• 친구에게 화를 내서는 안 돼요. (chinguege hwareul naeseoneun an dwaeyo.) = "You must not get angry at a friend."

Vowel stems are the simplest case. A stem with a final consonant looks longer on the page but follows the same rule.

Stems Ending in a Consonant

A consonant stem takes -아서는 or -어서는 directly, with no linking vowel. Choose by the last vowel: 앉 has ㅏ, so 앉아서는. 먹 has ㅓ, so 먹어서는. 읽 has ㅣ, so 읽어서는.

• 아무 데나 앉아서는 안 돼요. (amu dena anjaseoneun an dwaeyo.) [아무 데나 안자서는 안 돼요] = "You must not sit just anywhere."

The spelling keeps the final consonant, but the sound moves to the next syllable: 앉아서는 is spoken [안자서는]. This follows the standard pronunciation rules (표준 발음법) of the 국립국어원 (National Institute of Korean Language) for linking a final consonant to a following vowel.

Two more groups need a short note, because learners expect trouble with them.

Do Irregular Stems, Adjectives and Nouns Change?

Irregular stems change before -아/어서는, because 아 and 어 begin with a vowel. Check each type with the polite form you already know:

• ㄷ-irregular: 듣다 becomes 들어서는. 남의 말을 몰래 들어서는 안 돼요 = "You must not secretly listen in on other people."
• ㅂ-irregular: 돕다 becomes 도와서는, and 어렵다 becomes 어려워서는.
• 르-irregular: 모르다 becomes 몰라서는, and 누르다 becomes 눌러서는.
• ㅡ-stems: 쓰다 becomes 써서는.
• ㄹ-stems: 만들다 becomes 만들어서는. The ㄹ stays.

The pattern is used mostly with verbs. A few adjectives appear in rules and standards, where the meaning is "must not be...": 시험이 너무 어려워서는 안 돼요 (siheomi neomu eoryeowoseoneun an dwaeyo) = "The exam must not be too hard." Nouns are rare. With the forms clear, one sentence can be built step by step.

Worked Example: Building a Sentence with -아/어서는 안 되다

-아/어서는 안 되다 grammar works by turning an action into a rule. Follow the steps with the idea "You must not smoke on the street."

1. Start with the verb. The verb is 피우다 (to smoke). Its dictionary form ends in 다.
2. Find the stem. Drop 다. The stem is 피우. It ends in the vowel ㅜ.
3. Choose 아 or 어. The last vowel is ㅜ, not ㅏ or ㅗ, so choose 어. 피우 + 어 = 피워.
4. Add 서는. 피워 + 서는 = 피워서는.
5. Add 안 되다 in the formal style. For a sign, use 안 됩니다. Keep the space between 안 and 됩니다.
6. Rebuild the sentence. Put the place and the object first: 길에서 담배를 피워서는 안 됩니다. (gileseo dambaereul piwoseoneun an doemnida.) = "You must not smoke on the street."

You will know it worked when the place (길에서) and the object (담배를) stay in front and the last two words are 안 됩니다. The most common slip at this step is to drop 는, as in 피워서 안 됩니다. Always keep the whole 서는. The sentence is now a rule, but the pattern has limits on tense and sentence type.

Which Tenses and Sentence Types Work with -아/어서는 안 되다?

-아/어서는 안 되다 works in the present, in statements and in questions. The change happens on 되다, the last word. The verb before 서는 never changes for tense. The table shows the forms for 버리다.

The formal 됩니다 is spoken [됨니다] (doemnida), with ㅂ before ㄴ spoken as ㅁ. The contraction 선 replaces 서는 in speech and shortens the sound. 그렇게 해선 안 돼요 (geureoke haeseon an dwaeyo) [그러케 해선 안 돼요] means "You must not do it like that."

• 그렇게 해선 안 돼요. = "You must not do it like that."
• 시간을 낭비해서는 안 됩니다. (siganeul nangbihaeseoneun an doemnida.) = "We must not waste time."

A past form such as 안 됐어요 is rare. If you want to say that someone should not have done something, speakers more often say -지 말았어야 하다 ("should not have done"). The pattern is also not used for direct commands such as "Do not go." For a request aimed at one person, use -지 마세요. The most useful contrast for a learner is next to the everyday prohibition.

How Is -아/어서는 안 되다 Different from -(으)면 안 되다?

-아/어서는 안 되다 states a firm, general prohibition, while -(으)면 안 되다 states an everyday one. Both mean "must not," but the first sounds like a principle or a public rule, and the second sounds like a rule for a particular situation.

Both patterns contain a condition-like idea ("if you do X" or "by doing X"), but -아/어서는 안 되다 frames the action itself as wrong, while -(으)면 안 되다 sounds like a practical rule for the situation. The full rules for the everyday pattern are on the page for -(으)면 안 되다.

A second neighbour is -아/어도 되다, which gives permission. It is the positive counterpart of any "must not" pattern: 앉아도 돼요 ("You may sit"). A third neighbour is -지 않다, the long negative, which only says "do not" with no sense of rule. These contrasts lead to the smaller variations of the pattern itself.

Variations and Edge Cases of -아/어서는 안 되다

-아/어서는 안 되다 has four common variations: a set phrase, a written style, a general subject and a short spoken form. Each one appears in real conversation.

• 그래서는 안 돼요. The set phrase 그래서는 안 돼요 (geuraeseoneun an dwaeyo) means "You must not do that." It reacts to something someone just said or did, and it sounds firm but not harsh.
• Written plain style. In essays and reports, the last word is 안 된다 or 안 되며: 거짓말을 해서는 안 된다 ("One must not lie"). This is the usual form in rules and ethics.
• A general subject. The subject is usually everyone or no one in particular. Compare 약속을 어겨서는 안 돼요 ("People must not break promises") with 오늘은 약속을 어기면 안 돼요 ("Don't break your promise today"), which fits one situation.
• The spoken short form. Speech shortens 서는 to 선: 해선 안 돼요, 말해선 안 돼요. The meaning is the same. It is common in conversation.

The pattern also works with a modifier to describe a thing: 해서는 안 될 일 (haeseoneun an doel il) means "something that must not be done," where 될 is the future modifier of 되다. This lesson covers the firm prohibition used in rules and moral talk, not every way to forbid. The mistakes below show where the boundary causes real errors.

What Mistakes Do Learners Make with -아/어서는 안 되다?

Learners most often get the strength, the particle or the tense wrong. Four mistakes appear again and again.

1. Using it for small everyday rules. "No shoes inside" can sound too heavy with -아/어서는. Use -(으)면 안 되다 for ordinary house or classroom rules.
2. Dropping 는. 버려서 안 돼요 does not work as a prohibition. Without 는, 서 reads as "because" or "and then," so the sentence sounds like "It doesn't work because you threw it away."
3. Putting the past on the main verb. 했어서는 안 돼요 is wrong. The verb before 서는 stays in its basic stem form.
4. Mixing it up with -아/어서 meaning "because." 비가 와서 못 갔어요 ("Because it rained, I couldn't go") uses 서 for a reason. In -아/어서는 안 되다, the 서 is joined to 는 and carries no reason.

A quick check for any firm prohibition: Is the stem followed by -아/어서는? Is 는 present? Is the final word 안 되다, with a space? If all three answers are yes, the sentence is sound.

The practice set below tests these four points directly.

Practice and Answers

Try the four items before you read the answer key. Cover the answers with your hand.

1. Choose the form. Complete: 거짓말을 ___ 안 돼요. (해서는 / 하여서)
2. Fix the error. A learner writes 쓰레기를 버려서 안 됩니다 on a sign. Correct it.
3. Translate. "You must not touch other people's things carelessly." Use 만지다 and the polite style.
4. Write your own. Write one rule for a classroom or an office using -아/어서는 안 돼요. Then write it as a formal sign with 안 됩니다.

Answer key

1. 해서는. The 하다 verb contracts to 해, then 서는 is added.
2. 쓰레기를 버려서는 안 됩니다. The sign needs the full 서는.
3. 남의 물건을 함부로 만져서는 안 돼요. (namui mulgeoneul hamburo manjyeoseoneun an dwaeyo.) [나믜 물거늘 함부로 만져서는 안 돼요] (the particle 의 is also often said [에]: [나메]) = "You must not touch other people's things carelessly."
4. Answers vary. Check that 서는 is kept and the formal version ends in 안 됩니다. An example: 수업 중에 떠들어서는 안 돼요 → 수업 중에 떠들어서는 안 됩니다.

If you can complete items 1 to 3 without the key, you can use the pattern in conversation. Next, the lesson hands off to related patterns.

What Should You Learn Next after -아/어서는 안 되다 Grammar?

After -아/어서는 안 되다 grammar, learn the everyday prohibition, then the way to forbid with a request. This lesson teaches the firm prohibition and does not cover everyday rules, requests or obligation in depth. Those live on their own pages.

1. Start with -(으)면 안 되다, which is the everyday prohibition.
2. Continue with -지 마세요 don't, which is the request form, and -지 않다 negation, which is the long negative.
3. Return to Obligation and Permission to see every lesson in this set, or to the Intermediate Grammar list for other B1 patterns.

With these steps, -아/어서는 안 되다 becomes automatic: take the stem, add 아/어서는, and finish with 안 됩니다 or 안 돼요.

Frequently Asked Questions

What does -아/어서는 안 되다 mean?

-아/어서는 안 되다 means "must not" or "should never." It states a firm prohibition, often a rule or a moral principle. 거짓말을 해서는 안 돼요 means "You must not lie," and 쓰레기를 버려서는 안 됩니다 means "Littering is not allowed." It sounds more serious and general than the everyday -(으)면 안 되다.

How do I attach -아/어서는 안 되다 to a verb or adjective?

Take the stem and add -아서는 if the last vowel is ㅏ or ㅗ, otherwise -어서는, and 해서는 for 하다 verbs. 앉다 gives 앉아서는, 버리다 gives 버려서는, and 하다 gives 해서는. Then add 안 되다 in the speech level you need. It is used mostly with verbs, with only a few adjectives in rules.

Can -아/어서는 안 되다 be used in the past tense?

It is almost always present tense, because rules and principles speak about now and always. A past form 안 됐어요 exists, but it is rare. To say that something should not have been done, speakers more often use -지 말았어야 하다. The main verb before 서는 never takes a past marker: 했어서는 안 돼요 is wrong.

Is -아/어서는 안 되다 used in speech or only in writing?

It is used in both, but it has a formal and written flavour. Signs and notices use 안 됩니다, and essays use 안 된다. In speech, it is serious in tone and often shortened to -아/어선 안 돼요, as in 그렇게 해선 안 돼요. Everyday rules between friends usually use -(으)면 안 돼요.

What is the most common mistake with -아/어서는 안 되다?

The most common mistake is using it where -(으)면 안 되다 is more natural. A small everyday rule such as "no phones at the table" sounds too heavy with -아/어서는. A second mistake is losing 는: 버려서 안 돼요 is not a prohibition. A third is confusing it with -아/어서 ("because" or "and then"), which has no negative meaning of its own.

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