"남자가 저의 메시지를 읽습니다."
Translation:The man reads my message.
-neun and -ga are both Subject marking particles. When you are a beginning learner you usually do not understand the subtle difference, so I guess this is why many language learning apps and sites use them synonymously. They can't really be translated as you probably know. If you want to read up on that I suggest this site: https://www.howtostudykorean.com/unit1/unit-1-lessons-1-8/unit-1-lesson-2/