Learn Croatian: Pitam našeg novog vodiča gdje su kupili onaj suvenir blizu katedrale. - I ask our new guide where they bought that souvenir near the cathedral.

In a reported (indirect) Wh- question, the question word like gdje (where) acts as the first element of its clause. Croatian enclitics like the auxiliary verb su must take the second position in their clause, which is known as the Wackernagel rule. Therefore, su slots in immediately following gdje.

Why does vodič change to vodiča but suvenir stays as suvenir, given they are both masculine direct objects?

Both are in the accusative case, but Croatian makes a distinction between living (animate) and non-living (inanimate) masculine nouns. Because a guide (vodič) is a living person, its accusative form takes the -a ending, which looks identical to the genitive case. Meanwhile, the non-living suvenir takes no ending, looking identical to its nominative case.

Why do the adjectives take the forms našeg novog?

These adjectives modify vodiča, which is in the animate masculine accusative form. Adjectives describing living masculine objects in the accusative case must agree by taking the -og or -eg ending.

Why does the sentence use katedrale instead of the dictionary form katedrala?