"The singer sings at the park."
Translation:가수가 공원에서 노래합니다.
는/은 places emphasis on the singer (ie; the singer is ____) and 가/이 is a vague subject marker (here the main point of the sentence is singing at the park, 가/이 just tells you WHO sings at the park) now if someone were to ask you what the singer does, you would respond with '가수는.....' because the singer is the main focus. Emphasis is an important aspect of agglutinative languages and it takes some time to understand the nuances of it, especially for a native English speaker as english is an analytic language.
가수가 공원에서 노래 합니다 ... i can't remember exactly the other sentence (was sthg like: I sing in the street or at the library ...) but: here, duolingo said -> you have an extra space (between 노래 and 합니다) ... I did this extra space 'cause duolingo said that there has to be one (corrected the other sentence before) so ... what is right and what is wrong?