"There are almost no dogs."
Translation:개가 거의 없습니다.
and the particles strike again... why can't i say 개는 in this occasion? (only 개가 is accepted) (what are the nuances between both options here?)
It depends on nuance honestly. In this statement it's just a general fact that there are almost no dogs wherever you are. For most general statements like this you use 가/이
To use 은/는 you would need to be specific like saying "as for dogs, there are not many" or something like that since 은/는 is used for comparing and changing subjects as well.
At least this is the kind of guide i follow.
I've tried to follow logic like this, but the Duolingo answers really seem to be random about it. I suppose it's geared more for users using the multiple choice blocks than for people typing answers.
가 is used for when a subject is newly talked about. 는 is used when continuing to talk about such. So if you are talking about a house and then change the subject to dogs it would be 개가 and if if you continue to talk about the dogs 개는.