"I have olives."
Olivae = THEY, the olives: a nomin. plur. 1st declension form, used as a subject: The olives please me (I like the olives): Olivae mihi placent. The olives are on the table: Olivae sunt in mensa.
Olivas = THEM, the olives, an object form, accus. plur. Livia buys olives in the market: Livia olivas in foro emit.