Learn Croatian: Je li kombinacija za sef u zagonetki? - Is the combination for the safe in the riddle?

The base noun is zagonetka (riddle). Usually, in the locative case, the ending -ka changes to -ci due to a sound change rule called sibilarization. However, nouns ending in a consonant plus -ka (like -tka or -čka) often skip this change because the resulting consonant cluster is awkward to pronounce. Therefore, zagonetki is the standard and most common form.

Why does sef look like it is in the nominative form here?

It is actually in the accusative case. The preposition za (meaning "for" in this context) requires the accusative. Because sef (safe) is a masculine inanimate noun, its accusative form looks exactly like its nominative form. If it were a living thing, it would take the -a ending.

The current lesson is about splitting words around a preposition (like ni u čemu). Why doesn't that happen with u zagonetki?

That preposition splitting rule only applies to negative and indefinite pronouns that start with the prefixes ni- or i- (such as nitko, išta, or ikoji). Because zagonetka is a regular noun and not one of those special pronouns, the preposition u simply stays in front of it as a separate word.

Could I say Da li je kombinacija... instead of ?