Learn Croatian: Sredinom mjeseca medicinski konvoji putuju širom regije. - In the middle of the month, medical convoys travel across the region.

The preposition širom (across / throughout) always requires the genitive case. In this lesson, you are learning that it indicates spatial distribution—meaning the travel happens all over or across the region, which changes regija to regije.

Could I use diljem instead of širom in this sentence?

Yes, diljem regije means exactly the same thing. Both širom and diljem are used to express spatial distribution and both take the genitive case. While some strict Croatian grammar guides prefer diljem when used as a preposition, širom is extremely common in everyday speech and you will hear both used interchangeably.

Why does the sentence start with sredinom instead of using a preposition like u (in)?

In Croatian, it is very common to use the instrumental case of certain nouns to express a point in time without needing any prepositions. You will frequently see this pattern with početkom (at the beginning), sredinom (in the middle), and krajem (at the end).

Why is mjeseca in the genitive case right after sredinom?

Just like in English where we say "the middle of the month", Croatian uses the genitive case to show what the "middle" belongs to. Whenever you use time expressions like sredinom, početkom, or krajem, the noun that follows must answer "of what?" and will therefore be in the genitive.