"There are apples in the grass."
Translation:В траве яблоки.
I think that in the first sentence лежит is fundamental to convey the idea that the girl is lying on the grass, and not standing on it (на траве стоит девочка).
If that makes sense, apples would not require the use of something specifying their position on the surface because of their rounded shape. After all, as far as my understanding goes, you say that something стоит when it is in a more vertical position (like a glass of water), while something лежит when it is more horizontal (like a tray).
Oh, and as you can say that the apples are in (в) the grass instead of on (на) it, the use of these words become unnecessary if you go for the first option (apples in a tall grass).
How correct is it, native/advanced speakers?
I know that there are plenty of resources to know what case of word to use outside of Duolingo; but if DL isn't going to make any available on their site, they really need to work on the translations they suggest to users. For example i put "В траву яблоки" because траву was the translation suggested by the hints. This is annoying because if you struggle to remember when to use what cases (like I do) then marking answers like that correct just reinforces bad grammar.
Edit: For context "В траву яблоки" was accepted under the assumption it was a typo as of Feb 2020