-아/어야: Only If / Must (Necessary Condition) | Korean Grammar

-(으)면 says "if X, then Y" and leaves it there — X is merely one way to get Y. -아/어야 says something stronger and more specific: "only if X can Y happen." It marks X as the indispensable prerequisite — the thing that must be true for Y to be possible. English blurs these together under "if," but Korean keeps a clean line between a sufficient condition (-(으)면) and a necessary one (-아/어야). Getting this line right is what separates "if you study, you'll pass" from "you'll pass only if you study."

Building the form: vowel harmony

-아/어야 attaches to a verb or adjective stem and follows the same vowel harmony you already know from the -아/어요 present tense:

Stem vowel ㅏ or ㅗ → 아야: 가다 → 가야 (가 + 아야 contracts), 오다 → 와야, 좋다 → 좋아야, 살다 → 살아야.
Any other stem vowel → 어야: 먹다 → 먹어야, 있다 → 있어야, 마시다 → 마셔야, 읽다 → 읽어야.
하다 → 해야 (the usual irregular 하 → 해).

If you can already form the -아/어요 present, you can form -아/어야: it's the same stem-plus-vowel step, ending in 야 instead of 요.

열심히 공부해야 시험에 합격해요.

yeolsimhi gongbuhaeya siheome hapgyeokaeyo

Only if you study hard will you pass the exam.

비자가 있어야 들어갈 수 있어요.

bijaga isseoya deureogal su isseoyo

You can only enter if you have a visa.

The core meaning: necessary, not merely sufficient

This is the whole page in one contrast. Look at the same situation framed two ways:

돈이 있으면 살 수 있어요.

doni isseumyeon sal su isseoyo

If you have money, you can buy it. (money is one way — sufficient)

돈이 있어야 살 수 있어요.

doni isseoya sal su isseoyo

You can only buy it if you have money. (money is required — necessary)

The first sentence with -(으)면 presents money as sufficient: have it, and buying is possible — but it leaves open that there might be other ways too. The second, with -아/어야, presents money as necessary: no money, no buying, full stop. It's the difference between "if" and "only if / not unless." Whenever your English carries that "only if / you have to / it takes X to…" force, -아/어야 is the ending you want.

날씨가 좋아야 소풍을 가요.

nalssiga joaya sopung-eul gayo

We'll only go on the picnic if the weather's good.

손님이 다 와야 문을 닫아요.

sonnimi da waya muneul dadayo

We only close once all the guests have arrived.

약을 먹어야 나아요.

yageul meogeoya naayo

You'll only get better if you take the medicine.

Notice the shape of the main clause in these: it typically states a result or possibility — 합격해요, 들어갈 수 있어요, 나아요 — the thing that becomes attainable once the prerequisite is met. The prerequisite comes first, the payoff second.

💡

The English trigger words for -아/어야 are "only if," "not unless," "you have to X to Y," and "it takes X to Y." All four mark X as required, not optional. If X is merely one sufficient route, use -(으)면 instead. For a focused decision guide, see -(으)면 되다 vs -아/어야 하다.

Reinforcing with 만: "only if you actually…"

To sharpen the necessity even further, Korean often adds the "only" particle 만 right onto the ending: -아/어야만. This underlines that the prerequisite is the sole path — "only if you genuinely do X."

노력해야만 성공할 수 있어요.

noryeokaeyaman seonggonghal su isseoyo

Only if you actually make an effort can you succeed.

물을 많이 마셔야 건강해요.

mureul mani masyeoya geonganghaeyo

You have to drink plenty of water to stay healthy.

The 만 is optional — 노력해야 already means "only if you make an effort" — but it adds emphasis, a spoken insistence that there is no other way.

💡

The prerequisite can itself be a negative: 서두르지 않아야 실수를 안 해요 ("only if you don't rush will you avoid mistakes"). -아/어야 marks whatever must hold as the gate on the result — including a "must-not-happen."

Word order: prerequisite first, payoff second

-아/어야 always states the prerequisite clause first and the unlocked result second. English can front the result ("we can meet, only if I have time"), but Korean fixes the order: state the required condition, then what it makes possible.

시간이 있어야 만날 수 있어요.

sigani isseoya mannal su isseoyo

We can only meet if I have time.

예약을 해야 자리가 나와요.

yeyageul haeya jariga nawayo

You only get a table if you make a reservation.

Internalize this fixed order: whenever you want to say "…, only if…", flip it in your head to "[condition]-아/어야 [result]" before you speak.

Two important offshoots (handled elsewhere)

-아/어야 is the root of two very high-frequency patterns you should know exist, even though they get their full treatment on other pages:

1. Obligation: -아/어야 하다 / 되다 ("must / have to"). Add 하다 or 되다 to the bare -아/어야 clause and you get the standard way to say "must." This is so common it has its own obligation page — here just note that 가야 해요 ("I have to go") is literally "only-if-I-go [is it okay]," the same necessity logic completed by 하다.

지금 출발해야 기차를 탈 수 있어요.

jigeum chulbalhaeya gichareul tal su isseoyo

We can only catch the train if we leave now.

2. Regret: 았/었어야 (했다) ("should have"). With a past marker, -아/어야 turns into "should have [but didn't]" — a counterfactual regret. Again a Modality offshoot, but worth recognizing here.

어제 일찍 잤어야 했는데.

eoje iljjik jasseoya haenneunde

I should have gone to bed early yesterday (but I didn't).

Common Mistakes

Mistake 1: Using -(으)면 where the meaning is strictly "only if." This is the core error: it silently downgrades a requirement to a mere option. If a ticket is required to enter, -(으)면 loses that force.

❌ 표가 있으면 들어갈 수 있어요.

Weakens the meaning — this says 'if you have a ticket,' not 'only with a ticket.'

✅ 표가 있어야 들어갈 수 있어요.

pyoga isseoya deureogal su isseoyo

You can only get in if you have a ticket.

Mistake 2: Forgetting vowel harmony. A ㅏ/ㅗ stem must take 아야, not 어야.

❌ 날씨가 좋어야 소풍을 가요.

Wrong harmony — the ㅗ stem 좋 takes 아야: 좋아야.

✅ 날씨가 좋아야 소풍을 가요.

nalssiga joaya sopung-eul gayo

We'll only go on the picnic if the weather's good.

Mistake 3: The 하다 harmony trap. 하다 becomes 해야, never ×하야.

❌ 열심히 공부하야 합격해요.

Wrong — 하다 is irregular: 하 → 해, giving 공부해야.

✅ 열심히 공부해야 합격해요.

yeolsimhi gongbuhaeya hapgyeokaeyo

Only if you study hard will you pass.

Mistake 4: Leaving -아/어야 dangling as a standalone "must." The bare "only if" clause needs a main clause. To say "I have to go" on its own, you must complete it with 하다/되다.

❌ 저 지금 가야.

Incomplete — a bare -아/어야 clause needs a main clause; for standalone 'must,' add 해요/돼요.

✅ 저 지금 가야 해요.

jeo jigeum gaya haeyo

I have to go now.

Key Takeaways

-아/어야 marks the necessary condition: "only if X can Y," where X is required, not just sufficient. Contrast the merely-sufficient -(으)면.
Form it with vowel harmony, exactly like the -아/어요 present, ending in 야: 가야, 먹어야, 해야.
The main clause states the result or possibility unlocked by meeting the prerequisite.
-아/어야만 adds emphatic "only if you actually…"; -아/어야 하다/되다 is the "must" obligation pattern; 았/었어야 했다 is "should have."
Watch the vowel harmony (좋아야, not ×좋어야) and never leave the clause dangling as a standalone "must."

Now practice Korean

Reading grammar gets you part of the way. The exercises are where it sticks — free, no signup needed.

Start learning Korean→