-아/어야 되다/하다

This pattern is fundamental for expressing obligation, duty, or necessity in Korean. It's equivalent to 'must', 'have to', or 'should' in English. Both 되다 and 하다 can be used, and they are largely interchangeable, though 되다 is more common in everyday speech.

Conjugation Rules:
The form depends on the last vowel of the verb or adjective stem.

1. Stems with last vowel ㅏ or ㅗ: + -아야 되다/하다

• 가다 (to go) → 가 + 아야 되다 → 가야 되다/하다
• 보다 (to see) → 보 + 아야 되다 → 봐야 되다/하다
• 앉다 (to sit) → 앉 + 아야 되다 → 앉아야 되다/하다

2. Stems with other vowels (ㅓ, ㅜ, ㅡ, ㅣ, etc.): + -어야 되다/하다

• 먹다 (to eat) → 먹 + 어야 되다 → 먹어야 되다/하다
• 읽다 (to read) → 읽 + 어야 되다 → 읽어야 되다/하다
• 만들다 (to make) → 만들 + 어야 되다 → 만들어야 되다/하다

3. Stems ending in 하다: → 해야 되다/하다

• 공부하다 (to study) → 공부해야 되다/하다
• 운동하다 (to exercise) → 운동해야 되다/하다

되다 vs. 하다
While often interchangeable, there's a slight nuance. 되다 can feel like an external or natural requirement ('it has to be done'), while 하다 can imply a stronger sense of personal will or intention ('I must do it'). In most daily conversations, you will hear 되다 more frequently.

Tense and Formality:
This pattern can be conjugated into different tenses and politeness levels.

• Past Tense: 했어야 됐어요 / 했어야 했어요 (I should have done it)
• Formal: 해야 합니다 (I must do it)
• Informal: 해야 돼 (I have to do it)