I'm not sure I find Ruth's explanation particularly useful. The answer depends on whether you are using these as adjectives (don't use "a"/"an") or as nouns (do use "a"/"an"). He is American. Is this an adjective or a noun? Answer: adjective, maybe he was born and raised in Japan and is therefore Japanese, but currently holds US citizenship. He is an American (noun). They are American. Is this a noun or an adjective? It's an adjective, the plural (they) noun is "Americans": "they are Americans". This follows for all nationalities: European Europeans, Canadian Canadians, Kenyan Kenyans EXCEPT in the case of -ese (Japanese). While Japaneses is technically correct (it is in the dictionary), adding -s to -ese sounds wrong in English and so Japanese, Maltese, Chinese etc. all stay the same. To further complicate matters -ese words can't be used with a- or an- unless the -ese word is an adjective. I am Japanese. They are Japanese. Both of these can be nouns or adjectives, and where they are nouns they are automatically either singular or plural depending on whether they are used with I (singular), you (singular), you (plural) etc. I am a Japanese, he is a Japanese, she is a Japanese. These are all incorrect. I am a Japanese person (the "a" attaches to person (I am a person) and therefore as with the description above can be used), he is a Japanese person (correct), she is a Japanese citizen (again correct because "a" attaches to "citizen" and not Japanese, she is a citizen). Does this answer your question?

Japanese is both singular and plural, "My boss is Japanese", "Hayao Miyazaki is Japanese." "The ambassadors were Japanese.", "The Japanese have one of the world's leading economies." Nation on its own is singular, plural nations, i.e. "The League of Nations." American and European are singular. "An American in Paris" or "the man was European." Plural = +s.

Hi. Thank you for the comment!

Both statements are correct however the most common would not use the "a". Japanese is used for singular and plural. American and European are singular. To make them plural you would add an "s". Hope this helps. Ruth