Well, actually it could be both. Kind of.
The thing is that when you say en kaffe, you're talking about a cup of coffee (en kaffe = en kopp kaffe). However when you're talking about anything else regarding coffee, it's ett kaffe. For example if you're drinking coffee at a friends house and say "det här var ett gott kaffe!" (this was a good coffee!) you're talking about the brew or type of coffee itself. However if you're saying "det här var en god kaffe! (this was a good coffee!)" it means the same as "det här var en god KOPP kaffe" (this was a good CUP OF coffee!).
So basically, I think (not sure) that "en kaffe" is an abbreviation of "en kopp kaffe" and thus technically incorrect. However no swede (at least where I'm from) would even blink.
The same goes for tea by the way. "En kopp te" (a cup of tea), but "ett te" (a [kind of/pot of] tea).
I thought this was really weird, but then I thought about it and it's actually the same as in Dutch... "de koffie" and "een (kopje) koffie". Makes you think about your own language, too. The same as it's strange to translate 'like' with 'tycker om' - but in Dutch it would be 'leuk vinden' which is also two words, I just don't think about it when I'm thinking in English..
You could refer to either. If you say "ditt kaffe" I would interpret it as meaning that the person you're talking to usually makes coffee (i.e. "you make good coffee") or that you've tasted their cup and said it (sort of like "hey, yours is better than mine!").
I think ditt kaffe sounds better since it's referring to the beverage within the cup, but a native could be caught using either one and no one would care. So it's not black and white.
If "kaffe" is an ett-word, I don't understand why to say "en kaffe". I am ok with "en kopp kaffe", because "kopp" is an en-word, but if I don't use the word "kopp", I would say for example "Jag skulle vilja ha kaffe" (without the article), or "Vill du ha kaffe?", or "Tack för kaffet". In Italian "caffè" is a word of masculine gender: "a coffee" is "un caffè". "A cup of coffee" is "una tazza di caffè" (actually we say "una tazzina" = "a small cup"), because "tazza" is a word of feminine gender, but we don't say "vorrei una caffè" (I would like a coffee) at all! We can only say "un caffè" or "una tazza/tazzina di caffè" (not "una caffè"). So Swedish works differently, is that right?
"Kaffe" is a mass noun, but it still makes sense to talk about the coffee, which is "kaffet" in Swedish, so you are right, it is a neuter noun.
With "ett kaffe", you refer to one sort of coffee. "Jag skulle vilja ha kaffe" (without the article), or "Vill du ha kaffe?" are perfectly normal sentences. I guess you ask for "en kaffe" to stress that you want one cup only.
Some fun facts are that "en latte" does not mean a glass of milk, but a cup of "caffè latte" and that we say en cappuccino - två cappuccino :).
Tack för ditt svar. "En latte" betyder "un caffellatte" (inte "latte" = "mjölk") och man säger "En cappuccino; två cappuccino". Det är intressant! På italienska säger man "un cappuccino; due cappuccini". I've noticed that foreign people don't inflect Italian words (when they are linguistic loans). For example they say "bravo" (a masculine singular adjective in Italian) even referring to a woman or a group of people (instead of "brava" and "bravi"). "Jag skulle vilja ha kaffe" (without the article) in Italian literally means "vorrei del caffè" (as an uncountable noun), whereas "ett kaffe" means "un caffè" (one coffee), but the difference is quite subtle. :-)
I think it should be, but it's never used. See en kaffe refers to a cup of coffee, and ett kaffe refers to a brew/batch/type/way of making it (the liquid or the powder, to put it short), however we'd never talk about the cups in plural like that.
If you explicitly need to pluralize the cups of coffee, you would say "kaffekopparna" or "kopparna" (which actually means the cups the coffee is in rather than the cups of coffee)".
Say you're moving from one table to another though. You could either ask your friends to bring all the cups, or just "ta med kaffet".
din/ditt - singular item belonging to one person. dina - multiple items belonging to one person. er/ert - singular item belonging to a group of people. era - multiple items belonging to a group of people.
din and er are used with en-words. For example you'd say "din sko". ditt and ert are used with ett-words. For example you'd say "ditt hus".
dina/era can be used with both en- and ett-words, but only in their plural forms.
Hope that explained all the differences in a clear way!
Again a slightly clumsy translation to English (pretty rare in this app I must say) it shouldn't end up "I drink your coffee" but either "I drank your coffee" (past tense). Or "I will drink your coffee" (statement imperative). The answer could be a sort of question but again it's clumsy. Hope this helps and follows on from eating the dogs food.
This is not clumsy or a mistranslation, "jag dricker ditt kaffe" does mean "I drink your coffee".
"I drank your coffee" would translate to "Jag drack ditt kaffe".
"I will drink your coffee" (which I believe is future tense, not imperative? Isn't imperative used for commands?) is "Jag kommer (att) dricka ditt kaffe".
If the sentence was changed to a question, it would be "Dricker jag ditt kaffe?" or "Do I drink/Am I drinking your coffee?"