I've never actually studied the language, just learned from speaking with relatives and such, but I think you're basically correct. "Je" and "bio" will look a bit different depending on the object it refers to.
Ja sam bio
Ti si bio
On je bio
Ona je bila
Oni su bili
Mi smo bili
Not 100% sure about the spelling. Also it may defer a bit between the languages (Croatian, Serbian, etc.)