"I laugh in the room."
Translation:저는 방에서 웃습니다.
Why -에서 instead of -에?
It is 에서 because 웃다 is an action verb.
You might be thinking its 에 since 방 is a location. It is a location BUT 웃다 determines the marker. There is an action verb being done which is 웃다. So 에서 has to be used.