This is the 1999th most frequent Korean word.
방안에
"방안에" means "inside the room."
In this sentence, 방안에 indicates the location inside the room where the desk is situated.
방안에 책상이 하나 있어요.
There is a desk in the room.
Here, 방안에 specifies everything that is contained within the room to be organized.
방안에 모든 것을 정리해야 합니다.
Everything inside the room needs to be organized.
In this sentence, 방안에 describes the action of entering inside the room as the location.