"개가 공을 돌려줍니다."
Translation:The dog returns the ball.
I'm not understanding the difference by your explanation, since "the dog grabbed the ball in its mouth and then returned it" would be a textbook definition for "fetch" or "retrieve." How else could a dog possibly fetch or retrieve a ball? I'm not saying there isn't a difference based on the Korean, simply that its not clear by your explanation or Duolingo's context-free sentence.
I think what he's trying to say is fetch and return are contextually different.
Fetch implies the dog does not yet have the ball. He is going to obtain it. Then once he obtains it he will bring it back.
Return implies the dog already has the ball in his possession and is now brining it back.
Fetching is a two step process. Going to get the ball, then bringing it back.
Returning is just the latter half of that process.
It would have been nice to learn how to make the casual form before this lesson since it's needed for it, however it's unavailable at this level. Whyyyyy, I'm already confused 24/7 about this language, I don't need to be even more confused ㅠㅠ