Can, must, may, must not
A2 능력·의무·허락·금지 34 / 68
In short
Four everyday “modal” ideas each have their own frame, and three of them lean on 되다 (doeda, to be okay / to work): can = stem + -(으)ㄹ 수 있어요 (할 수 있어요, can do); must = stem + -아/어야 돼요 (해야 돼요, have to do); may = stem + -아/어도 돼요 (해도 돼요, may do); must not = stem + -(으)면 안 돼요 (하면 안 돼요, must not do). Watch two things throughout: the spaces in 할 수 있어요, and the spelling 돼요, never *되요.
The rule
Each frame is just a stem-ending you already know (-(으)ㄹ, -아/어, -(으)면) plus a fixed little tail (수 있다, -야 되다, -도 되다, 안 되다). Build the front part the normal way and the tail never changes.
Step 1 - Ability: -(으)ㄹ 수 있다 / 없다
Verb stem + -(으)ㄹ 수 있어요 = can, -(으)ㄹ 수 없어요 = cannot. Here 수 is a bound noun meaning a way / possibility, so it takes the prospective modifier ending -(으)ㄹ from lesson 24: -ㄹ 수 after a vowel (and after a ㄹ 받침), -을 수 after any other 받침.
- 하다 -> 할 수 있어요 (hal su isseoyo) - can do it
- 먹다 -> 먹을 수 있어요 (meogeul su isseoyo) - can eat
The negative -(으)ㄹ 수 없어요 overlaps with 못 from lesson 14: 못 해요 (mot haeyo) and 할 수 없어요 (hal su eopseoyo) both mean can’t do it. 못 stresses being blocked in the moment - and mind the space, because the single word 못하다 is a separate dictionary entry meaning to be bad at something; -(으)ㄹ 수 없어요 states plain impossibility and is a touch more formal. But 못 has no positive twin - to say you can, you must reach for -(으)ㄹ 수 있어요. (A different “can” - knowing how to do something - is -(으)ㄹ 줄 알다, saved for lesson 52.)
Step 2 - Obligation: -아/어야 되다 (and -아/어야지요)
Stem + -아/어야 돼요 = have to / must. Build the -아/어 form (lesson 10), add -야, then 돼요: 가다 -> 가야 돼요 (gaya dwaeyo), 먹다 -> 먹어야 돼요 (meogeoya dwaeyo), 하다 -> 해야 돼요 (haeya dwaeyo). Spoken Korean prefers 되다; writing and formal speech prefer 하다 - 가야 해요 (gaya haeyo) - with no change in meaning.
A close cousin, -아/어야지요 (often shortened to -아/어야죠), is a soft self-reminder or gentle nudge: 이제 가야지요. (Ije gayajiyo.) - We really should get going now.
Step 3 - Permission: -아/어도 되다
Verb stem + -아/어도 돼요 = may / it’s okay to. Same -아/어 form, this time with -도: 가도 돼요 (gado dwaeyo, may go), 먹어도 돼요 (meogeodo dwaeyo, may eat), 봐도 돼요? (bwado dwaeyo?, may I look?). As a question it asks permission; as a statement it grants it. Two interchangeable variants swap 되다 for 괜찮다 or 좋다: 봐도 괜찮아요? (bwado gwaenchanayo?) and 봐도 좋아요 (bwado joayo) mean the same thing.
Step 4 - Prohibition vs no obligation - do not confuse them
These two both carry a “not”, and English blurs them, so decide which the situation means before you translate.
- Must not = Verb stem + -(으)면 안 돼요 - a wall. It is the -(으)면 conditional (lesson 29) plus 안 돼요: literally if you do it, it won’t do. 가면 안 돼요 (gamyeon an dwaeyo) - you must not go.
- Don’t have to = negate the verb inside the permission frame: 안 + Verb + -아/어도 돼요, or stem + -지 않아도 돼요 - a free pass. 안 가도 돼요 (an gado dwaeyo) = 가지 않아도 돼요 (gaji anado dwaeyo) - you don’t have to go.
So “you don’t have to go” is 안 가도 돼요, NOT 가면 안 돼요 (which forbids going).
Step 5 - Answering, and two orthography traps
To a -아/어도 돼요? question, grant with 네, 돼요 / 네, 그러세요 (Ne, geureoseyo, yes, go ahead) / 그럼요 (Geureomyo, of course), or refuse with 아니요, 안 돼요 / 아니요, -(으)면 안 돼요.
Two things trip up nearly everyone: 수 and 있다 are separate words, so 할 수 있어요 has two spaces; and 돼요 is 되 + 어요 contracted - there is no *되요.
Forms & patterns
The four frames at a glance (shown on 하다):
| Meaning | Pattern | Example on 하다 |
|---|---|---|
| can, be able to | stem + -(으)ㄹ 수 있어요 | 할 수 있어요 (hal su isseoyo) |
| cannot | stem + -(으)ㄹ 수 없어요 (= 못 + verb) | 할 수 없어요 (hal su eopseoyo) = 못 해요 |
| have to, must | stem + -아/어야 돼요 (spoken) / -아/어야 해요 (written) | 해야 돼요 (haeya dwaeyo) |
| may, it’s okay to | stem + -아/어도 돼요 | 해도 돼요 (haedo dwaeyo) |
| must not | stem + -(으)면 안 돼요 | 하면 안 돼요 (hamyeon an dwaeyo) |
| don’t have to | 안 + verb + -아/어도 돼요 / stem + -지 않아도 돼요 | 안 해도 돼요 (an haedo dwaeyo) |
-(으)ㄹ 수 있다 - the ending picked by the 받침:
| Stem ends in | Ending | Worked example |
|---|---|---|
| a vowel | -ㄹ 수 있어요 | 가다 -> 갈 수 있어요 (gal su isseoyo) - can go |
| a 받침 other than ㄹ | -을 수 있어요 (an irregular stem reshapes first and then takes what its new shape calls for, lesson 17: 듣다 -> 들을 수, 돕다 -> 도울 수) | 읽다 -> 읽을 수 있어요 (ilgeul su isseoyo) - can read |
| ㄹ 받침 (the ㄹ absorbs the ending) | -ㄹ 수 있어요 | 만들다 -> 만들 수 있어요 (mandeul su isseoyo) - can make |
-아/어 harmony for the 되다 frames (obligation and permission use the same 아/어 form):
| Stem-final vowel | -아/어야 돼요 (must) | -아/어도 돼요 (may) |
|---|---|---|
| ㅏ, ㅗ -> -아 | 가다 -> 가야 돼요 (gaya dwaeyo) | 가다 -> 가도 돼요 (gado dwaeyo) |
| any other vowel -> -어 | 먹다 -> 먹어야 돼요 (meogeoya dwaeyo) | 먹다 -> 먹어도 돼요 (meogeodo dwaeyo) |
| 하다 -> 해 | 하다 -> 해야 돼요 (haeya dwaeyo) | 하다 -> 해도 돼요 (haedo dwaeyo) |
| ㅗ contracts (보 -> 봐) | 보다 -> 봐야 돼요 (bwaya dwaeyo) | 보다 -> 봐도 돼요 (bwado dwaeyo) |
Asking and answering for permission:
| Question | Granting | Refusing |
|---|---|---|
| 들어가도 돼요? (Deureogado dwaeyo?) - May I go in? | 네, 돼요. / 네, 그러세요. (Ne, geureoseyo.) / 그럼요. (Geureomyo.) | 아니요, 안 돼요. (Aniyo, an dwaeyo.) / 아니요, 들어가면 안 돼요. |
Examples
- 저는 한국어를 조금 할 수 있어요. (Jeoneun hangugeoreul jogeum hal su isseoyo.) - I can speak a little Korean.
- 오늘은 바빠서 갈 수 없어요. (Oneureun bappaseo gal su eopseoyo.) - I can’t go today because I’m busy.
- 저는 매운 음식을 못 먹어요. (Jeoneun maeun eumsigeul mot meogeoyo.) - I can’t eat spicy food.
- 여기에서 사진을 찍어도 돼요? (Yeogieseo sajineul jjigeodo dwaeyo?) - May I take photos here?
- 아니요, 여기에서는 찍으면 안 돼요. (Aniyo, yeogieseoneun jjigeumyeon an dwaeyo.) - No, you must not take photos here.
- 오늘 안에 숙제를 다 해야 돼요. (Oneul ane sukjereul da haeya dwaeyo.) - I have to finish all the homework today.
- 내일은 학교에 안 가도 돼요. (Naeireun hakgyoe an gado dwaeyo.) - I don’t have to go to school tomorrow.
- 이제 자야지요. (Ije jayajiyo.) - I really should go to sleep now.
Common mistakes
- 할수있어요. (Halsuisseoyo.) -> 할 수 있어요. (Hal su isseoyo.) - 수 is a bound noun and 있다 an auxiliary, so 띄어쓰기 puts a space on each side of 수.
- 가도 되요? (Gado doeyo?) -> 가도 돼요? (Gado dwaeyo?) - 되 needs a vowel ending: 되 + 어요 contracts to 돼요. There is no *되요.
- 가면 안 돼요. (meaning “I don’t have to go”) -> 안 가도 돼요. (An gado dwaeyo.) - -(으)면 안 되다 forbids (must not); don’t have to is 안 …-아/어도 되다. The two are opposites.
- 가을 수 있어요. (Gaeul su isseoyo.) -> 갈 수 있어요. (Gal su isseoyo.) - A vowel stem takes -ㄹ 수; -을 수 is only for a 받침 (먹을 수).
- 가아도 돼요? (Gaado dwaeyo?) -> 가도 돼요? (Gado dwaeyo?) - The ㅏ-stem 가- + -아도 contracts obligatorily to 가도; the uncontracted *가아도 is never written.
Tips
- The 되/돼 test. 돼 is short for 되어, so wherever 되어/되었 would fit, write 돼/됐. A quick check: swap in 하다 - where 해 sounds right, use 돼; where 하 sounds right, use 되. Thus 안 돼요 (like 안 해요) but the dictionary form 되다 (like 하다). This fixes native writers too.
- Wall vs free pass. Picture -(으)면 안 돼요 as a wall (forbidden) and 안 -아/어도 돼요 as a green light (optional). English “don’t have to” and “must not” both hide a “not”, so pick the meaning first, then the frame.
- No one-word “can”. Korean has the short adverb 못 for can’t, but nothing short for can - the positive is always -(으)ㄹ 수 있어요. Pair them: 못 해요 <-> 할 수 있어요.
- Speak 되다, write 하다. In conversation obligation is -아/어야 돼요; in writing and formal speech it becomes -아/어야 해요. Permission and prohibition likewise favour 되다 in speech (해도 돼요, 하면 안 돼요).