I need to know why do we say "schwarzen Kaffee, bitte" and not "schwarzer Kaffee, bitte". To me, it should be "schwarzer Kaffee", however, I am being told it is the other way around.
I assume you are using these phrases in the context of placing an order.
is a short form of
“Schwarzer kaffee, bitte!” could be used to tell the black coffee to behave. ;)
Trying to emphasize the grammar background, I offer this answer:
It is because the line is an answer to the question "What may I serve to you?" or "What do you want?".
The reply is in the case accusative, answering "whom or what?", to which you reply "schwarzen Kaffee", while "schwarzer Kaffee" would be nominative, answering to "Who or what?".
In English, the adjective "schwarz"/"black" is the same in both cases, but in German, it isn't.
In contrast, if you are asked "What is your favourite drink"/"Was ist ihr Lieblingsgetränk?", you'd answer with a nominative, like "Schwarzer Kaffee".