Yes, of course it's grammatically correct, "coffee" is a noun. The indefinite article "a" + any bare noun is always grammatically correct -- with the minor exception that you'd add an "n" if the noun starts with a vowel sound: "an elephant."
If you mean that it's idiomatic, yes, not only is it idiomatic, it's what most people ordering coffee would say: "I'd like a coffee, please. Large, black."
Well, technically, it's not grammatically correct because "coffee" is a mass/noncount noun, which doesn't license an indefinite article. However, it is idiomatic as a short form of "a cup of coffee." As an aside, your usage of "bare noun" is incorrect; a bare noun is a noun that does not take a determiner, which means it is not preceded by any article.
It's not easy to distinguish, but those characters are used in the stressed part of the word to show which part of the word you pronounce with more force. For example, if you were to say "¿Donde está mi papel?" or "Where is my paper?", you would emphasize the last character in está. This is used sometimes just to indicate how to pronounce the word, but can also be used to distinguish two words from one another. For example, 'está' means 'is' while 'esta' means different things in different sentences but is mostly used as 'this' or 'this one'.
Evertime I'm sure I understand un and una, I get a sentence like this.
I typed it in Spanish and it said i typed in English. So I typed in English thinking maybe the instructions were wrong. No. Didnt matter if i typed the phrase in English or Spanish it always said that I typed it in English and wouldn't check it or move on. I finally just typed "xxx" and check. At least then it told me i was wrong and will now let me move on.
The Voice recognition software keeps slaughtering what I am saying, some times in a very humorous combination of random words put together. (But the absolute worst word that it NEVER gets right, EVER not even once, no matter how I might vary the pronounciation is :"un". ) It's very irritating! Can you fix it? Thanks.
I don't think I've ever seen sugar spelled "suger" in English. Are you talking about another language? "Suger" isn't really an English word. There is, however, a French statesman named Suger, and he was one of the earliest patrons of Gothic architecture, and he’s widely credited with popularizing the style. Fascinating!