Only if

The pattern -아/어야 is used to express a necessary condition. It means that the action or state in the second clause can only happen if the action or state in the first clause is fulfilled. It's stronger than a simple 'if' (-(으)면) and carries the nuance of 'only if' or 'one must... in order to...'.

Formation

Attach -아/어야 to the stem of a verb or adjective.

Stems ending in ㅏ or ㅗ: Add -아야

보다 (to see) → 봐야

가다 (to go) → 가야

Stems ending in other vowels: Add -어야

먹다 (to eat) → 먹어야

있다 (to exist) → 있어야

하다 verbs: Becomes -해야

공부하다 (to study) → 공부해야

사과하다 (to apologize) → 사과해야

직접 봐야 믿을 수 있어. / 먹어야 힘이 나지. / 공부해야 합격해. / 돈이 있어야 살 수 있어.

Learners often confuse -아/어야 with the general conditional -(으)면 ('if'). The key difference is necessity.

-(으)면: General Condition

This indicates a simple 'if A, then B' relationship. It suggests one possible condition among many.

돈이 있으면 살 수 있어요. (If you have money, you can buy it.)

Nuance: Having money is one way to buy it. Maybe you could also use a credit card or get it as a gift. The condition is not exclusive.

-아/어야: Necessary Condition

This indicates that A is the only condition that will lead to B. Without A, B is impossible.

돈이 있어야 살 수 있어요. (You must have money to be able to buy it.)

Nuance: Money is the absolute requirement. No other options will work. This is a much stronger statement.

돈이 있어야 살 수 있어요. / 선생님이 계셔야 시작해요. / 사과해야 용서해요. / 눌러야 열려요.

-아/어야 often appears in set phrases or can be strengthened with other particles for emphasis.

With -지/죠

Adding -지 (casual) or -죠 (polite) to the final verb creates a sense of stating an obvious truth, like saying 'of course' or 'naturally'.

먹어야 힘이 나지. (Of course you have to eat to get energy.)

먹어 봐야 맛을 알죠. (You have to taste it to know the flavor, obviously.)

With -만 for Emphasis

To further emphasize that the condition is the one and only way, you can add the particle -만 (only) to form -아/어야만.

선생님이 계셔야만 시작할 수 있어요. (We can start only if the teacher is here.)

먹어야 힘이 나지. / 먹어 봐야 맛을 알지. / 잃어 봐야 소중함을 알아. / 직접 가 봐야 알아요.

The -아/어야 part of the pattern does not change for politeness. Instead, the formality of the entire sentence is determined by the verb ending in the final clause.

Casual Language (반말)

Use a plain form ending like -아/어 for speaking with close friends or those younger than you.

잃어 봐야 소중함을 알아. (You only realize its value after you lose it.)

Polite Language (해요체)

Use the -아요/어요 ending for most daily conversations with colleagues, new acquaintances, or people older than you.

잃어 봐야 소중함을 알아요. (You only realize its value after you lose it.)

직접 가 봐야 알아. / 직접 가 봐야 알아요. / 지금 가야 제시간에 도착해. / 지금 가야 제시간에 도착해요.