"Tu ai iubit-o pe ea?"
Translation:Did you love her?
Why does the "o" go after the verb in this case and not before it? Tu o ai iubit pe ea?
That's the direct complement (complementul direct) who answers to the questions "pe cine", "ce" (who, what). Here, the complement is expressed through a personal pronoun in the female gender (her, that can be just implied). This happens in the past perfect with the female gender. You always put it at the end like in the example above. The male complements come in front. "Tu l-ai iubit pe el - Tu îl ai iubit pe el". I hope I was clear enough. :D Maybe someone can explain it better, but thats the gist of it.