Yes. It should be all-American athlete.
Yes, because Celtic is a name. Just like American is capitalized, Celtic is capitalized.
Yes; "American" is always capitalized.
Yes. African American should be capitalized.
It depends. If it is the name of a specific class, then it would be capitalized: American History 101. If it is in sentence that only generally speaks about American history, then only "American" would be capitalized: American history.
If you are using it as a proper name yes. If you are using it as an adjective then no.
Yes, if it a subject called American History. Otherwise, just American is capitalized.
The word "All-American" as a noun is a proper noun referring to a person, an athlete. So it is a concrete noun.
You should capitalize "American" when referring to the nationality or citizenship of someone from the United States, as in "American citizen." However, you do not capitalize it when referring to general concepts or things relating to America, such as "the American culture."
American should always be capitalized but not the child. It should be- America n child
Yes, because Celtic is a name. Just like American is capitalized, Celtic is capitalized.
"American people" is capitalized when it is used as a proper noun referring to the citizens of the United States as a collective group.
Yes, the "A" in American should be capitalized when referring to the nationality or cultural aspects of the United States. So it should be "American customs."
Yes; "American" is always capitalized.
no
Yes. The word American is always capitalized in English, as are other nationality adjectives. Although a case could be made for "American cheese" if "french fries" is lowercased.
No, except at the beginning of a sentence because it is not a proper noun but American should always be capitalized.
American is a proper noun. It can also be an adjective, as in "American cheese." A rule of thumb: proper nouns are capitalized and common nouns are not capitalized.