When we want to say should or must do something, we conjugate korean verbs by changing the verb into its present form (아/어/여) and adding ‘야 하다/되다’ to the end.
Whether the ending is 야 해요 or 야 돼요 doesn’t matter that much, they mean the same.
Examples
그를 봐야돼요! = You should see him!
운정해야해요. = You have to drive.
What are passive verbs? A passive verb is a word that indicates that an action was done in the past. In English, we say something is/are/was done to make it passive. In Korea, there are several ways to make a passive verb. Keep in mind that while passive verbs are used often, it is often more natural just to use the active form of a verb. For example, to find rather than is found, to close rather than is closed, etc. Also, passive verbs also can mean the possibility of taking that action.
1. When used with an action verb, (아/어/여)지다 turns the verb into a passive verb.
주다 (to give) + (아/어/여)지다 = 주어지다 = to be given = can be given
자르다 (to cut) + ㄹ라 (irregular) + 지다 = 잘라지다 = to be cut = can be but
2. Generally, when the verb ends in ㅎ, 이다 is the ending.
When the verb ends in ㄱ,ㄷ,ㅂ 히 is the ending.
When the verb ends in ㄹ, 리 is the ending.
When the verb ends in ㄴ,ㅁ,ㅅ,ㅊ 기 is the ending.
놓다 (to put down) + 이 = 놓이다 = to be put down = can be put down
먹다 (to eat) + 히다 = 먹히다 = to be eaten = can be eaten
밀다 (to push) + 리다 = 밀리다 = to be pushed = can be pushed
씻다 (to wash) + 기다 = 씻기다 = to be washed = can be washed
3. Only 하다 verbs use this ending. 하다 verbs become 되다 verbs when they become passive.
이용하다 (to use) – change into 되다 = 이용되다 = to be used = can be used
연구하다 (to research) – change into 되다 = 연구되다 = to be researched = can be researched