Learn Croatian: Moramo premjestiti sofu pred prozor. - We must move the sofa in front of the window.

The preposition pred takes the accusative case when it shows motion toward a destination. Since we are moving the sofa to a new spot in front of the window, it requires the accusative. If the sofa were already sitting there and not moving, we would use the instrumental case (pred prozorom) to show its static location.

Since prozor is in the accusative case, why doesn't its ending change?

Prozor is a masculine noun. For masculine nouns, the accusative form looks exactly the same as the nominative (dictionary) form if the object is inanimate (non-living). It only gets an -a ending if it refers to a living being.

Why is sofu used instead of sofa?

The sofa is the direct object receiving the action of the verb premjestiti (it is the thing being moved). Because sofa is a feminine noun ending in -a, it changes to -u in the accusative case.

Could we use the imperfective verb premještati instead of premjestiti?

You could, but it changes the focus. Premjestiti is perfective, meaning the focus is on successfully completing the action of moving the sofa. If you used premještati, it would imply an ongoing or repeated action, like pushing the sofa around the room for a while.