Why there's no article with 'Mexican' and 'an' with American in sentence:

"She’s Mexican. But she’s married to an American." (Pimsleur English course)

Answer

"Mexican" is an adjective here, "American" is a noun. Both the words, "Mexican" and "American" can be either adjective or a noun. So all of the following are valid:

• "She is Mexican."


• "She is a Mexican."


• "She is American."



• "She is an American."

Not all nationalities are like this though. (Usually the ones ending in -an work as above.) For example, "English" and "French" are only adjectives. The noun form would be "a Frenchman" or "an Englishman", but these words are not used much. It's more common to say "a French guy" or an "an English woman" or something along those lines.

Side note: In the US, in certain contexts it can be considered rude to refer to someone as "a X" e.g. "a Mexican". Saying "a Mexican man" would be more polite.