"A passerby helped me."
Translation:지나가는 사람은 나를 도와줬어요.
지나가는 사람이 나를 도와줬어 (casual form)
지나가는 사람이 저를 도와줬어요 (polite form)
sound more natural though.
지나가는 사람은 나를 도와줬어요. = this kind of polite and casual form mix can happen in a sentence rarely between a mum and a child conversation (a child can talk to his or her mum with some pseudo formality since they are super close though they are not buddies)