"Mr. John is American."
Two questions here:
1- I missed the "wa" particle; is it really necessary? I thought that in Japanese you could avoid pointing things if it was understood by the context (and I think here it is).
2- Shouldn't "amerikajin" carry any particle? Or is it because the verb "desu" doesn't need because it is not actually a verb?
"desu" and "-masu" verb forms are generally used with people you don't know well or someone who is in any sense your senior. So I guess the equivalent in English would be calling your teacher or boss by their first name (when they haven't expressed it to be okay) and speaking too casually in a formal setting. Keep in mind though that the rules are more bent for foreigners. I've often been told and heard that they don't really care when foreigners make mistakes about using honourifics because they are doing their best (and even Japanese make mistakes when using the more difficult honourific forms).
Yeah i thought that at first, that it wasn't in the other examples on duolingo, but i think the difference is they were talking about yourself "MY name is maria" "I am John, I am American", whereas here you are talking about someone else. Super confusing to give you the same two names over and over, both as a hypothetical name for yourself and again for hypothetical other people, if you ask me.
How 'bout accepting 米人 and 米国人 for "american person" ? It is technically correct I think, that's how I learnt it and I've been checking it in a japanese dictionnary. If someone could confirm/tell me I'm wrong, I think the meaning of 米国人 hits closer to the U.S. than アメリカ人.
So, you're right (congrats!), but there's another point to be aware of. It's only used in writing. What's more is that it's only for being formal or honorific. You'd never say 「私は米国人(べいこくじん)です」. I just asked my Japanese roommate and he said that it'd be super strange to here someone say that, but he uses that for writing formal documents or when being super polite about a high-superior's country...