Learn Croatian: Ostavio je odličan dojam na roditelje. - He made an excellent impression on the parents.

In Croatian, the standard phrase for making an impression is ostaviti dojam, which literally translates to "leaving an impression." You cannot use the verb for "make" (napraviti or činiti) in this context.

Odličan dojam is the direct object here. Why does it look like it is in the nominative case?

Because dojam (impression) is a masculine inanimate noun. In Croatian, masculine nouns that do not represent living beings have an accusative form that looks exactly like the nominative. The adjective odličan matches this rule and stays in its base form too.

What case is roditelje, and why is it used after the preposition na?

Roditelje is the accusative plural of roditelj (parent). The preposition na takes the accusative case here because the impression is being directed onto someone. If na were describing a fixed location, it would take the locative case instead.

Could we start this sentence with the pronoun on (he) to make it On je ostavio...?

Yes, you can say On je ostavio odličan dojam na roditelje. However, Croatian typically drops subject pronouns because the verb itself already tells us who is doing the action. The pronoun on would only be added if you specifically wanted to emphasize that he (and not someone else) made the impression.