"The cat is in a tree."
Translation:Кошка на дереве.
I wouldn't say that the cat is literally "inside" the branches, but is rather among the branches. English is heavily idiomatic, and therefore allows for quite a bit of colloquial flexibility. It's perfectly acceptable to say that a cat is "in" a tree without literally meaning that the cat is inside the tree, or imbedded in the tree.
Prepositions are a little weird when translating across languages. I've been having trouble with Russian prepositions myself. You just have to figure out how different languages apply their prepositions. Coming here to ask questions helps.
In English, the conditions for which we would say "the cat is on the tree" are different, and somewhat unusual. For instance, if a cat is climbing a tree and has stopped half way up the trunk, you could get away with saying that the cat is "on" the tree.
Another, slightly more likely and less specific scenario would be if a tree has fallen over. The tree is laying sideways on the ground, and the cat is sitting "on" the tree. However, if the cat is within the branches if this fallen tree, we would still say "in". It would probably be something like "the cat is in those tree branches".
It's very context sensitive and isn't exactly intuitive, but I hope this helps.
What you want to look for is motion, or lack thereof. For instance, stationary prepositions such as in, at, on, above, below, etc. will take the ablative case. Prepositions that indicate motion such as into and onto will take the accusative case.
Now that I think about it, looking up Russian declensions and conjugations might not be a bad idea.
Are you sure you mean "ablative"? None of the cases that Russian uses are called ablative. There is an ablative case used in some other languages that roughly refers to "motion away from" - that doesn't line up with Russian usage. The case you're referring to is usually called prepositional case or locative case.
Does "Cat in tree" make sense to you? You can't judge his Russian sentence using English rules. His sentence makes sense. It's just that the team decided that only sentences with neutral word order would be accepted, otherwise they'd have to enter every possible sentence with those words. They talk about it here: https://www.duolingo.com/comment/13955228
So don't patronize him with your faulty logic.
Actually, Beingfollowed has a point. While the word order may make sense in Russian, why does diesch want to change a valid Russian word order which parallels the English to force his/her own personal interpretation on the programmers of Duolingo?
It's like the numerous people studying other languages who insist on not using valid cognates, instead expecting Duo to accommodate their own personal flights of literary fancy with synonyms.
So there's a point to the question, although it might have been stated more diplomatically: Why does diesch want to change the word order when the "correct" answer is quite obvious and readily available?
This isn't a course in creative writing, it's a beginning language course.
The English sentence should be "the cat is on a tree" not "in". Makes it very misleading when translating the sentence into Russian.