"The boxes are on the desk."
Μετάφραση:Τα κουτιά είναι πάνω στο γραφείο.
A desk is something you sit at. An office is usually a building, used for working in, which usually contains many desks. In some cases, an office can also be a single room, which contains at least one desk for working at. But, desk and office both translate as γραφείο. The πάνω gives further context here as the boxes could not be on an office. To give an example using desk: "O διευθυντής είναι στο γραφείο του".