Learn Croatian: Moram ići oftalmologu, a zatim optičaru. - I must go to the ophthalmologist, and then to the optician.

In Croatian, when you are going to a person or their workplace (like a doctor or a friend), you typically use the bare Dative case without a preposition. So, ići oftalmologu means to go to the ophthalmologist. You might sometimes see the preposition k used here (ići k oftalmologu), which is also correct and slightly more formal, but the bare Dative is extremely common.

Wait, aren't destinations supposed to use the Accusative case? Why are these in the Dative?

You use the Accusative case (with prepositions like u or na) when your destination is a physical place or an institution, such as u bolnicu (to the hospital). But when your destination is a person, such as a doctor or a specific professional, Croatian shifts to using the Dative case to express going to see them.

Why is it a zatim instead of i zatim for "and then"?

The conjunction a is often used for transitions, not just contrasts like "but". When stringing events together in a sequence, Croatian naturally pairs a with time words to mean "and then" or "and next". Phrases like a zatim and a onda are the standard, natural-sounding ways to transition to the next step of a plan.