Learn Croatian: On je jučer prošetao kroz park. - He took a brief walk through the park yesterday.

The base verb šetati means 'to walk' or 'to stroll' as an ongoing, imperfective activity. Adding the prefix pro- makes it perfective (prošetati), which limits the action. It implies taking a short walk or completing a walk through a space, which perfectly matches the idea of taking a brief transit through the park.

I often see this verb used with the reflexive pronoun se. Why is se missing in this sentence?

Both šetati and prošetati can be used with or without the reflexive se. Adding se (as in prošetao se) focuses slightly more on the personal leisure of taking a stroll. Without se, the focus leans more toward the physical motion of walking from point A to point B. However, in everyday Croatian, the two forms are practically interchangeable.

Which case is the word park in, and why does it look exactly like the dictionary form?

The word park is in the accusative case here because it follows the preposition kroz (through), which always requires the accusative. It looks identical to the nominative (dictionary form) because park is a masculine inanimate noun. For masculine nouns representing non-living things, the accusative singular always matches the nominative.

If I wanted to emphasize that this happened yesterday by starting the sentence with jučer, where would the verb go?