"A man lost a cat."
Translation:남자는 고양이를 잃어버렸어요.
버리다 means to throw away.
잃다 and 잃어버리다 both mean to lose, but 잃다 is a lot less common and can only be used with abstract things, like memories or relationships. If you lose an object you have to use 잃어버리다.
Am I still not understanding the difference between 는/은 and 가/이, or was my answer (남자가) actually also correct but not accepted?