Learn Croatian: Djevojka s bijelim ogrtačem čeka masažu. - The girl with the white robe is waiting for a massage.

In standard Croatian, if someone is wearing a piece of clothing, you use u + locative (so, u bijelom ogrtaču). Using s + instrumental (s bijelim ogrtačem) technically implies she is physically carrying or holding the robe, though it is sometimes used colloquially to mean wearing it, similar to the English phrasing.

Why do we use the short preposition s instead of sa?

The short form s is the default. The longer form sa is only required when the following word starts with sibilant letters (s, z, š, ž) or certain tricky consonant clusters to make pronunciation easier. Since bijelim starts with a 'b', the short form s is used.

Why is there no preposition translating to 'for' before masažu?

While in English you 'wait for' something, the Croatian verb čekati typically takes a direct object. You directly 'wait' the thing or event, so masažu is put in the accusative case without needing any preposition.

Could the verb čekati take a case other than the accusative here?

Sometimes čekati can take the genitive case, but this is usually reserved for abstract concepts, periods of time, or when waiting for an unspecified amount of something. For a specific, definite event like masažu, the accusative case is standard.