Well, I think it's mainly because we're not in the XIV century. But surely if you want to use "ye", you could might as well start using these: http://en.wikipedia.org/wiki/Middle_English_personal_pronouns
To be less rude, in most places "ye" is considered archaic. When I was in Ireland last month, some people did use it, although it still seemed pretty rare to me there. Duolingo probably should accept it, but it is not common English, and Duolingo often does not accept more archaic sounding sentences until someone reports it specifically.
I believe Duolingo teaches "American" English. And in the US, we don't say, "Ye" very often. It's found in the Bible, and old English.
The only common examples of its use I can think of (outside of literature) is 1) the Christmas Carol, "O, Come All Ye Faithful" and 2) the expression "Oh, Ye of little faith." This phrase can be used to refer to someone's lack of religious faith. But it is also used when someone lacks (or does not have) faith in your ability to do something. IE: "John, I don't think you can do it" to which John would reply, "Oh, Ye of little faith!" Normally, it is used in a humorous manner.
Althought it accepts both "you are not A family" and "you are not family" both have a different meaning in english.
When someone says "you are not family" it usually implies that a single person is being accused of not being part of the family (this is used in drama movies and such and is not practical in everyday life)
I agree. 'You are not family' is different to 'You are not a family' and there's a place for both. I also accept that the latter is the correct English translation for the German 'Ihr seid keine Familie' but I'd like if a native speaker can help with this: How does one translate 'You are not family'? Would that be 'Ihr seid Familie nicht'?
I'm not one to trust on this, being a beginner and all (I've been trying to just do my best at giving dubious advice here and there), but you might be able to just use "bin" instead of stating "Ich bin" every time. I feel like I've heard that said before, also this is just a guess based on the fact that this same concept is accepted in Spanish (using "soy" instead of "yo soy" all the time).
Actually, I think you have to say "Ich bin" always, like in English (I asked the same to my German teacher and she told me to always use the pronouns like in "Ich bin"). I think Spanish is the only language where you can avoid using the pronouns all the time (I don't know if there are other languages where you don't have to use the pronouns neither)
I've always wondered if using the negative form like "you are no family" or "I have no chocolate" or "I am no boy" could also be considered correct? (Could it be that it's just too informal or so? I'm not a native English speaker and every time I have to translate negative forms, that word order comes to my mind)
AntoPiero, Yes, those sentences are correct. To me, they sound more formal, (rather than informal.) But it is much more common to say them like this: "You are not family" (instead of: "you are no family") "I do not have any chocolate." (instead of: "I have no chocolate") "I am not a boy" (instead of: "I am no boy")
It can be considered correct. It emphasises what the thing is that is negated, especially when it looks like something that it isn't. In "You are no family", it emphasises that the people being spoken to are specifically not a family, even if they look like one. In "I have no chocolate", it is emphasising the fact that chocolate is what the person speaking doesn't have. In "I am no boy", it emphasises that the person speaking is not a boy. In one iconic quote, someone says "That's no moon, that's a space station." Here this form is used to show that, despite looking like a moon, it isn't, and is actually a space station.
This is a pretty late reply, but I hope it'll be useful to you or other readers. I strongly disagree with bynny2015 below, saying "I have no .." in English tends to be more informal/dramatic rather than formal/correct.
Eg. you would say "I am a boy", this would be neutral, formal and correct.
If you say "I am no boy", this would be informal or dramatic, to emphasise a point. This might not be obvious to non-native English speakers, but a lot of languages like German, Swedish etc seems to use a version of "I have no" casually in daily conversation. This is often not the case for English speakers, say in UK or US.
Saying "I have no money" in US for instance, is similar to how some people would say "I ain't got no money", which is a slang/local usage that is understandable, but not generally regarded as formal.