Did you ever notice? (2019 December) this fantastic study feature!
In the app, before we tap on continue, when there's audio and word tiles, we can practice before continuing to next exercise within lessons.
I'm on Android, studying Korean from English today.:
--- Anyone notice it in Apple, or desktop apps? --- Also, say if you see it for other languages? --- and are you on free Duolingo or Plus subscription.
To form the use of "at" as a Place Particle, the 에 is missing 서. This would also be similar to "in", which would also be an acceptable answer I would think, since there is no other "in" being used within the remainder of the sentence. Is there another reason for the particle being half complete given this pedigree? My original answer was "There are only sixty-one books in the library."
The way I have been told 에 = location particle. 에서 = shows the location of an action. So if someone is doing something at the library such as 'I study at the library' 나는 도서관에서 공부해요. In this case, I don't think the 61 books are up to anything. The key I try to remember is if there is action involved.