"고양이가 의자에 앉아요."
Translation:The cat sits in the chair.
Prepositions (in almost any language that has them) always have quirks, and "in" vs "on" aren't the only strange ones in English. While there are general guidelines, some cases can't be explained. I (a native English speaker) would never say "She sits in the sofa" or "she sits in the couch", I would always use "on" for those. But I would say "she sits in a chair" about as often as "she sits on a chair"--both are correct, and I don't know of any guideline or rule for why "in" is acceptable for "chair" but not "couch" or "sofa", except possibly that the space where one would sit could be considered to belong to the chair in a similar way that the space where people sit in a canoe or other small boat would be considered as belonging to the canoe even though much of the space is above the canoe rather than inside of it.
A similar confusion happens with English-Italian translations for certain kinds of objects like tables and balconies. In Italian, the preposition "in" (which normally translates to the English word "in") is used to say someone is, what we would say in English, "on the table" or "on the balcony", and as best as I can tell, it's because "in" specifies locations, and in Italian, the table is thought of not just as a piece of furniture but as a space or location, and a balcony is thought of as another kind of room.
Other unexplainable (and inconsistent) uses of "in" and "on" in English are that we can say "on the bus" or "on a plane" or "on a train" or "on a cruise ship" to mean inside any of those things, but we can't say "on a car" to mean inside the car, we have to say "in the car" in that case. Any native English speaker will agree, but none can explain why.
It's actually not nice for LEARNING the things, but it is nice to repeat things after you learned them through other websites or apps. I'd recommend the website "How to study Korean" to you if you really want to know a lot about grammar - they really go into detail. If you want it shorter, you should maybe use other sources. I have the Korean From Zero! books and they break it down to the minimum while still teaching you everything you need to. It's really nice for both learning new things and repeating.
i do not like how duolingo seperates important lessons relating to grammar with things that seem relatively unimportant. do i really need to know specifically about food and animals before i can communicate that something will happen in the future? at least if its something fun for casual learners, please dont restrict my ability to view lessons that i want to prioritize.