Learn Croatian: Oni se odnose jedan prema drugome s poštovanjem.

In Croatian, when using the reciprocal phrase jedan drugi (each other) with a preposition, the preposition must split the two words. You cannot put it before the phrase like in English. You are literally saying one towards the other (jedan prema drugome).

Why does drugome take the dative case here while jedan is in the nominative?

In this construction, the first half (jedan) always stays in the nominative case. The second half (drugome) changes case based on the preposition right in front of it. Here, the preposition prema (towards) requires the dative case.

If se makes the verb reciprocal, why do we also need jedan prema drugome?

While the pronoun se often shows reciprocal action on its own, verbs that require a specific preposition to express their meaning, like odnositi se prema (to treat/behave towards), need the jedan drugi construction to attach that preposition and clearly show the target of the behavior.

Why is the preposition s used in s poštovanjem? Doesn't the instrumental case alone mean with?