Learn Croatian: Ulaznice se prodaju na blagajni. - Tickets are sold at the ticket office.

Here, se is used to create a passive voice. Instead of saying that someone specific sells the tickets, we say Ulaznice se prodaju to mean Tickets are sold. The focus is purely on the action happening to the tickets, not on who is selling them.

Since se is used to make it passive, why is the verb in the plural form (prodaju)?

Even though it acts like a passive sentence, grammatically in Croatian, ulaznice (tickets) is still the subject of the sentence. Because ulaznice is plural, the verb prodavati (to sell) must also be in the plural form: prodaju.

Why do we use the preposition na instead of u for at the ticket office?

In Croatian, certain locations, open spaces, and service counters take na instead of u (in). A ticket office, counter, or cash register (blagajna) is one of these words. So, you do things na blagajni (at the register or counter).

What case is blagajni in this sentence?

It is in the locative case. The preposition na followed by the locative case is used to show a stationary location, answering the question of where something happens. The feminine noun blagajna ends in -a, which changes to -i in the locative.