Same problem, and there's nothing particularly unusual about it in English. It is a more direct translation, and quite acceptable English, in my opinion! Ok maybe it sounds a bit like a line from a nursery rhyme, but I think it should be accepted as it's clear that the meaning of the Russian was understood.
Native English speaker here, and I have to agree with the others saying that it sounds perfectly natural to say in English. And even if it sounded unnatural, it should still be correct.
Edited to say: Also it should be translated this way because you can use "In the box is a ___" for most of the other phrases in this particular lesson.
Both are technically correct, but "жива" and "мертва" sounds better than "живая" and "мертвая".
Long adjectives are mostly used to describe a constant quality, while short adjective are better for describing the state in witch the object is right now.
I know the distinction is kind of blurry, especially with the word "dead" which is permanent either way. But still, we prefer to use the short form when contrasting life and death, and talking about the fact that someone has died and the long form when just descibing a dead person or an animal.
- "Мы не знаем, жив ли кот или мертв" - "We don't know, whether the cat is alive or dead"
- "Его бабушка мертва" - "His grandmother is dead" (as opposed to being alive. This sentence is not about her dead remains, but about the fact that she is no longer with us)
- "Он мертв, Джим" - "He's dead, Jim" (He might have survived, but no, the doctor confirmed, he's dead)
- "На дороге лежит мёртвая птица" - "There is a dead bird on the road"
- "Его мертвое тело нашли в реке через три дня" - "His dead body was found in the river three days later"
- "Там волк!" - "Не бойся, он мертвый." - "There's a wolf there!" - "Don't worry, it's dead" (It's not about the fact, that the wolf has died recently, but rather about the fact that a dead animal can't attack us)
I hope, that helps. Sorry for the grim examples :)