Because we are not talking about specific coffee, but coffee in general. One can think about it as 'some coffee', though I doubt that Duo accepts this as an answer. Ich trinke den Kaffee. would mean 'I drink the coffee'.
You need to use the accusative masculine definite article here: "Ich trinke den Kaffee."
You also wouldn't use it here, because the English sentence doesn't have "a" in it.
- "I drink coffee" = Ich trinke Kaffee (I drink coffee generally)
- "I drink the coffee" = Ich trinke den Kaffee (I drink a specific coffee)
- "I drink a coffee" = Ich trinke einen Kaffee (I drink one coffee, i.e. one cup or variety of it).
How do I say (I do not drink Kaffee)? My guess was simply (Ich trinke nicht Kaffee). But apparently it is wrong.
nicht and keine means the same but they differ in the context, in this case you would say ich trinke keine Kaffee, because keine applies to numbers, like ich habe keine Hunde (so you have "# 0" dogs) while nicht is for non measurable things, like es ist nicht gut, "that isn't good".
Good example, but "ich trinke keinen Kaffee"*. Remember the noun gender.
Can this be used only to mean "I drink coffee," in the sense of "I am drinking coffee at this very moment;" or can it also mean that "I drink coffee generally," as in "I am a coffee drinker?"
In real life you usually would be able to tell from the context since random strangers probably would not come up to you, say: "Ich trinke Kaffee." and leave. Instead, you may be speaking on the phone with a friend and ask them what they are doing, other than talking to you, and they could say "Ich trinke Kaffee". Or, if you are buying a gift basket fore someone you could ask "Trinkt er Kaffee?"-"Does he drink coffee?" to find out if he drinks coffee in general. :)
Not related to the sentence, (i don't know where to post ideas.) but i feel they should have something similar to this, but instead you translating instead of just typing the words you hear.
As you review/repeat the lessons (which Duolingo encourages you to do, as well as learning new lessons) a bit of variety is added in in terms of the tasks. So, they already do what you suggest.
General comments/suggestions/discussions should be put in the appropriate discussion forum: https://www.duolingo.com/discussion