"The backpack is in front of the desk."
Translation:배낭이 책상 앞에 있어요.
Particles that mark locations and positions depend upon what kind of verb you use.
In general, ~에 is a particle that marks locations or events. This is necessary for verbs like 있다 but can also be used with most verbs.
- 저는 학교 살아요 = "I live at school."
When you are using a verb of motion like 가다 ("to go") or 수영하다 ("to swim"), ~에 is used to mark destinations.
- 저는 학교에 걸어요 = "I walk to school."
Conversely, with a verb of motion the particle ~에서 is used to mark an origin position.
- 저는 학교에서 걸어요 = "I walk from school."