"You ask the teacher."
你问老师 should be fine. You are simply making the sentence singular. Also, you put 是 instead of 师 which is incorrect.
'You' can be plural or singular in English. The only way to tell is from context. Since there is not enough context in this sentence, it is impossible to tell, so both 'ni' an 'ni men' are correct translations
In this case it's both since we don't know if it's plural or singular in English
Yeah, the way the question is composed it makes it seem there is only one other person (ergo 你) not multiple.
it is correct, but in this case it meant "you" as a plural form. That's just the English language issue, and u shold report that
I think what is confusing is thinking it's a colloquial "you guys" used in place of "they" when it's actually a plural "you".
Please either change the english to be "You all ask the the teacher" or change the chinese to "你问老师." 谢谢你
11/20/17, still is only accepting the plural, 你们 in spite of the sentence being ambiguous in English, it seems to me that it would be correct for this to accept both 你 and 你们.
Duolingo insists on "you guys" to indicate plurality in answers. Maybe it could do us the same favor in questions. :)