Translation:Do you prefer the big box or the small box?
But at least duolingo is pretty easy to use. As not native English speaker my inferior English can complicate stuff a lot when people use not direct language.
Like prefer, does it mean just wich you like more or wich one do you take.
OK here it's irrelevant. But often teachers get lazy and write textbook were even English speaker get confused and for me it's an even bigger nightmare then.
Typically "prefer" is used when someone is asking you an opinion.
Either you are buying it and they are getting it for you (like a waiter taking your order at a restaurant), or they are buying for themselves and they want your opinion (your significant other asking which item of clothing you like more on them) .
When I said "There is a big box and a small box, which do you prefer", it marked it as wrong, and only marked it correct when I wrote "There ARE a big box and a small box, which do you prefer". Does that sound weird to anyone else, or do I need to brush up on my English grammar?
I think you are correct. One can't say "there are a", "a" is for singular (except when in "a lot of"). "Are" does not fit here because you're talking about one thing at a time: there is a big [box] and [there is] a small box. I'm not a native speaker, though. https://www.grammarly.com/blog/there-is-there-are/
If anyone is wondering. ookii and chiisai are "i" adjectives, but ookii and chiisai are commonly switched to 'na' adjectives when places directly before a noun. However, keeping them as an 'i' adjective when placed before a noun is STILL correct. "Ookii hako," and, "Ookina hako" both mean big box and are both correct.
I don't think this can answered. Every answer i put it changes one word and marks wrong so i change the word for next time and it still marks wrong with a new change.
Examples. Me: Which do you prefer a big box or small box? Duo: Which do you prefer the big box or the small box?
Me: The big box or the small box which do you prefer? Duo: A big box or a small box which do you prefer?
Me A big box or a small box which do you prefer? Duo: A big box and a small box which do you prefer?
This is getting really annoying. I've reported it three times now.
This may be just me, but if I wanted to get the nuance of "prefer," I would add a 「のほうが」。
「～のほうが」（～の方が） Literally means "The side of..." So if I wanted to compare two things, I could stick a のほうが on the one I wanted to draw attention to. I would also add a 「よりも」 to the one I wanted to compare it to.
このはこのほうが そのはこよりも 大きいです。
This box is bigger than that box.
You can also get away with replacing the のほうが with a simple は。
このはこは そのはこよりも 大きいです。 (same meaning)
Whatever comparison you can make in English, you can make in 日本語。 Just to show you that you can make the opposite emphasis; it doesn't have to be the biggest or best:
このはこのほうが そのはこよりも 小さいです。
So, to answer your question, I don't think "prefer" is wrong per se; I just think there are better ways to get that meaning.
Hope this helps!