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.