Let's see... ja idem u ured, ja idem u Zagreb... and it's used with accusative when the verb you're in transition much like German or Russian. When you're staying somewhere you're using the other case marking location: ja sam u Zagrebu, ja sam u uredu... And, no, you don't always use preposition "u" meaning "in" you can for instance use "na" meaning "on" with both accusative for transition or, location case meaning staying somewhere: ja idem na trg (going to the square), ja idem na otok (going to the island), ja idem na posao (going to the work)... ja sam na trgu (I'm on the square), ja sam na otoku (I'm on the island), ja sam na poslu (I'm at work)... "IZ" means "from" and it's not opposite to "U". You mostly use it when you say where are you from or where are you coming from: Ja sam iz Rusije (I'm from Russia), Izlazim iz dućana (I'm coming out of the store), Putujem iz Zagreba u Split (I'm traveling from Zagreb to Split). "I'm living in Dubrovnik in a house with garden" (Ja) živim u Dubrovniku u kući s vrtom. And if you need help with Croatian, just ask, I'm here to help... :-)