"Lampa jest za biurkiem."

Translation: The lamp is behind the desk.

[gbaldacci1309]

Is "biurkiem" taking the Instrumental Case because of "jest"? So "za" does not affect the object in this sentence?

[immery]

preposition always dictates the case. za can be followed by accusative or instrumental. If it's direction- it's accusative, if it's location it's instrumental, if it's not literal you need to learn phrases with preposition and case.