Learn Croatian: Zatvori vrata za sobom i usredotoči se na posao. - Close the door behind you and focus on work.

In English, we say "close the door behind you". But in Croatian, whenever the pronoun refers back to the subject of the sentence (the "you" doing the closing), you must use the reflexive pronoun sebe. Because it follows the preposition za meaning "behind", it takes the instrumental form sobom.

What case does the preposition za take here?

Here, za is followed by the instrumental case (sobom is the instrumental of sebe). When za is used to mean "behind" in terms of physical location or arrangement, it takes the instrumental case.

We just learned the full forms of sebe. Why do we use the short form se in usredotoči se?

The verb usredotočiti se (to focus) is a reflexive verb that naturally takes the short enclitic form se. You only use the full accusative form sebe when you want to heavily emphasize the object, which doesn't fit the natural flow of this general instruction.

Why is posao in the accusative case after the preposition na?

The phrase for "to focus on" is usredotočiti se na followed by the accusative case. Even though there is no physical motion involved, this specific verb and preposition combination always requires the accusative to show the target of your focus.