There are these related phrases:
- 켜다(turn on), 켜지다(come on), 켜져 있다(is on - a state).
- 끄다(turn off), 꺼지다(go out), 꺼져 있다(is off).
*켜지다/꺼지다 are passive forms of 켜다/끄다(transitive verbs).
Examples:
- I turned on the light: 나는 불을 켰다. ("불" means "light", or "fire").
- I turned off the light: 나는 불을 껐다.
- The light came on: 불이 켜졌다. (or 불이 들어왔다).
- The light went out: 불이 꺼졌다. (or 불이 나갔다).
- The light [TV] is on: 불이 [티브이가] 켜져 있다.
- The light [TV] is off: 불이 [티브이가] 꺼져 있다.
In a noun modifying phrase, 켜져 있는/꺼져 있는 is very often simplified to 켜진/꺼진.
- 불(이) 켜진 사무실들: Offices with lights on.
- 불(이) 꺼진 창: A lightless window. (불이 꺼져 있는 창 is used when more clarity/emphasis is needed).
- 꺼진 불도 다시 보자: Look again even when the fire appears out. (from a fire hazard awareness campaign)