Why does "Are you a Japanese?" sound wrong compared to "Are you Japanese"? As opposed to "Are you a Canadian?" and "Are you Canadian?", both of which sound fine.
Unfortunately, I don't know enough about how to ask this question to use meaningful tags, sorry.
English has no systematic way of naming people of a given nationality. Some nationalities have only an adjective ("he is a Chinese man"), some have an adjective and a special noun ("she is a Scottish woman/she is a Scot", "He is a Swedish man/He is a Swede"), and some have an adjective and a noun that are the same word ("she is a German/she is German")
In your question, you've discovered the first category (Japanese) and the third (German), and you've also touched on the only real rule in deciding between them...
Where the adjective ends with "-an", you can be reasonably confident in using it as a standalone noun too. For example, the following are all usable as the word in the two sentences "She was a(n) _____ woman" and "She was a(n) _____":
Be careful that you don't accidentally a word that has a more common meaning already: someone from the Dalmatia region of Croatia is indeed "a Dalmatian", but if you use that word, people will think you're calling them a dog.
For adjectives ending in "-ese", you can't use the word as a noun. There's either a special noun to describe the person (Faroese/Faroer), or none at all (Chinese, Japanese). There aren't many of these adjectives in English.
In cases where the adjective doesn't end in any of these, it's hit or miss.
Not "-an", but still the same word In many cases, the "official" noun is the same as the adjective, although this is an old rule, and today most of these will sound a little odd, archaic, or even insulting even though they're still technically correct (e.g., "an Argentine","a Basque", "a Swiss") - the only one I can think of offhand that doesn't sound strange is "Greek".
"A Basque" is also an example of the case where you need to be careful of not using a word that has another more common meaning (here, the kind of corset worn by women).
use the stem There's also a whole collection of special nouns, like "Swedish/Swede", "Finnish/Finn" where the noun is just the stem of the adjective. But be careful: "a Flemish man " is most definitely not a "Flem"
Preferred demonyms and their usages change over time. "A Japanese" was formerly a common and semantically neutral way to refer to an individual of Japanese origin, but it has been supplanted by "a Japanese person," so it sounds wrong now.
In the 1930s, the common and preferred term was "a Japanese," so it would not have sounded wrong to English speakers then. Consider the following examples:
These preferences will vary between demonyms of the same form; there is no rule in English that the -ese suffix can be used only in adjectives and not in nouns. For example, a person from Guyana is still commonly called "a Guyanese," as seen in Akola Thompson's 2021 article "Nine things to consider when dating a Guyanese." Thompson is from Guyana.
Both mean the same thing, but there's a subtle difference between the two phrases.
If I ask you, "are you Canadian?", you are likely to feel that I'm asking about your personal attribute. I'm interested in you, and perhaps want to know more about you.
But if I ask you "are you a Canadian?", it's more impersonal. I'm lumping you in with a group of those people. It's not directly insulting, and that might not be my intention, but it could feel that way.
Both involve stereotyping, but with the first one I'm trying to build my image of Canadians using you as a type, but with the second one I'm more likely to be applying my existing image of Canadians to you.
But note that this generalization doesn't always apply to all nationalities. I've known Scotsmen that would much prefer to be known as "a Scot" than as merely "Scottish" (though it can be a deliberate humorous affectation on their part). There's a long tradition of national pride in Scottish history and how "the Scots" resisted invasion and assimilation from Roman times until the present.
"Are you Japanese?" asks about the ethnic background of a person.
"Are you a Japanese?" asks whether the recipient of the question is a Japanese person.
Presumably it has already been established that the recipient of the question is a person. For that reason, this version of the question is partially superfluous, and so should be avoided.
