"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)
It should either be 도와줬어 or 저를 else the levels of formal speech are all mixed up
The two posts above regarding the use of the "I" pronoun in Korean and formality are not correct.
As most people learning Korean are aware, there are three levels of formality. 나/저 can both be used for 존댓말 비격식 (polite informal).