"I teach English to my Japanese friends."
As far as I understand it the "no" is being used to describe the type of friends. "What type of friends? The Japanese ones." It isn't describing two separate groups ("the friends" and "the Japanese people"). I am not sure how to write, "The friends of the Japanese people" though, so I can't help you there. Sorry!!! Hopefully this helped a little bit though.
日本人の友達に英語を教えます。 -- Duo: "Wrong." Doesn't like the 達 and 教 characters.
So I went for the safe option of writing it completely in hiragana...
にほんじんのともだちにえいごをおしえます。 -- Duo: "Wrong."
Can't win. It almost always accepts sentences written completely in hiragana. So I assume there must be an alternate spelling such as にっぽんじん that it's looking for (or maybe the default is misspelt such as にほんひと or some other crazy thing). XD
As you can see from the date underneath, it was over 2 years ago (April 2018) I wrote that reply to Miriam91979.
Back then the course was still on Tree 1.0 (the original version of the tree, which had only 40 skills). I completed Tree 1.0 almost a year earlier (May 2017) and always used the report feature very heavily. So I most likely already submitted a "my answer should be accepted" report for this sentence over three years ago now...
Further, as can be seen at the top of this sentence discussion, Duo's default Japanese translation for this sentence has since changed to "日本人の友達に英語を教えます", exactly the same as my first answer in that comment. So it's pretty obvious it accepts this answer now. But back when I wrote that comment, Duo's default Japanese translation was the following:
Btw, "whenever Duo rejects a correct answer, report" might not be entirely good advice. At least not in my situation where there were two rejected correct answers.
From what I've heard from course contributors, only the first report a user submits of each report type for each sentence actually gets received by the system. Even if it's a differently worded rejected answer, still only the first report of the "my answer should be accepted" type will work, unfortunately.
After the first one, any more won't actually go anywhere and will instead simply vanish into thin air as soon as you click submit. It's only once a contributor someday gets around to either accepting or rejecting the first report (of that type, from that user, for that sentence) that the system will finally once again be able to receive a new report (of that type, from that user, for that sentence).
Therefore, in my comment where I mentioned two correct answers that were rejected... If I had reported both rejected answers as "my answer should have been accepted", the second one would have been a complete waste of time. ^^;
(The first one would've probably been a waste of time too. Back when I wrote that comment was before Luke and Ehartz joined the contributor team. The original contributor team had seemingly mostly gone AWOL by that time, meaning it was very slow progress for reports to get processed back then. Therefore, I would've almost certainly had a "my answer should have been accepted" report for this sentence stuck in the system backlog already.)
In my Japanese classes, we were told that you do need の to combine two different nouns. It's not necessarily a possessive although it could be in some cases. It's possible that some nouns might be compound nouns without the の but I'm not sure. It's best to put it in between nouns.
OK, so as of today, May 14, 2020, it marked my "日本の友達に英語を教えます" as INCORRECT. Whereas, "日本の友達に電話をかけました" was marked CORRECT for "I called my Japanese friend".
Why? and, I think I will send a report.
also, "日本人の友達に電話をかけました" for "I called my Japanese friend" was marked INCORRECT the first time I typed it. ::Edit:: i just realized the first time, i actually put "日本語の友達に電話をかけました", which is definitely wrong.