The author uses a rhetorical question to emphasize that the man who works hard gains nothing. Translate "what profit does ... gain" as you did in [Ecclesiastes 1:3](../01/03.md). Alternate translation: "For the person who works so hard and tries in his heart to complete his labors under the sun gains nothing." (See: [[rc://en/ta/man/translate/figs-rquestion]])
This refers to things that are done on earth. See how you translated this in [Ecclesiastes 1:3](../01/03.md). Alternate translation: "on the earth" (See: [[rc://en/ta/man/translate/figs-idiom]])