Translation:What is inside the box?
Duolingo's other courses with this sentence reference Brasd Pitt in the comments, NEVER FAILS.
I am not sure whether は is required, but it does sound better.
To my understanding, に alone would just be 'in'. So 箱の中に何がありますか really is a straightforward "what is in the box?". Adding は to something turns the preceding part into the topic of the sentence, with が denoting the actual subject. That is more like "about the inside of the box (箱の中には), what is it?" Either translates to "What's inside the box?" though.
thank you for the explanation. I'm still adjusting to the "topic" of sentences with は although I don't really get what the difference is in just saying "what's in the box" vs "the inside of the box, what is it" because they sound like the same topic to me
From what I learned in my class, に and で can replace は in Japanese's grammar (where it doesn't with も). Basically, the topic becomes implied by the use of either of these particles. It's not farfetched because Japanese uses context for the topic all the time. Using ある can mean "there is" or "I have" depending on the context or if 私は is present.
I'm assuming that it's in the same idea that adding it anyway adds emphasis to the topic of the sentence, since that is what は is for. What I'm confused about is why が wouldn't be used for this purpose since that's what が exists for when not paired with です/だ.
I think this is such a good example to see how the Japanese particles work all together to build a sentece, naisu
It's a mistake. It should be pronounced as "hako". Only when は is used as a topic marker then it's pronounced "wa"
This sentence is more like "is there anything in the box" 何がある Means "do you have something?"
(箱には)何を入っている Is more like "what's in the box?" (箱 Would usually be left out of the sentence)
Well, I could tell you what's in the box, OR you could choose what's behind door #1.
Really depends on their usage; i.e. other characters that they're combined with and context. They can both mean "inside", but the former is more like the "center / middle" (of a location) and the latter more like "within" (e.g. a period), to name just two things.
Would "is there anything inside the box" require a different translation or is DL just being picky with their answers?
Also, wouldn't はこの中には何ですか be a more literal translation?
Yes, "Is there anything inside the box?" requires a different translation (as LeakyJam said). And はこの中には何ですか is grammatically incorrect; "to be" in the sense of "to be [somewhere] / to exist" is either いる（います）or ある（あります）, depending on whether it's alive or not. です is "to be [something]".
You can think so, but that's a different question in either language.
In the case of "Is there something in the box?" you don't know whether there is or isn't anything in there. Asking "What is in the box?" implies you already know there is something in there, just not what exactly.
You would usually get that from context. Or if someone asked "すべての箱（はこ）の中（なか）には何がありますか？” that would mean what is in all of the boxes. Although the word "はこ" cannot be pluralised, because the word "すべて” meaning "all" is mentioned, you can gather that there are multiply boxes.
So given the lack of context, you can't, right?
To clarify: In the question you have to choose either box or boxes. I chose boxes (before I even had time to see that box was an option), which was deemed incorrect.
If there isn't any context assume it's singular. Yes there are particular cases where this same sentence could be used when talking about multiple boxes. But it is safe to assume that this is one box and that is how it would usually be translated.