"My coffee cup is empty."
Translation:Mia taso de kafo estas malplena.
Why can't I turn coffee into an adjective and just say "kafa taso"? (Reported it in case it's just a missing alternative.)
I don't think that would be the only interpretation. The parentheses around 'made of' seem to suggest that 'made of' is just one possible reading. After all, a 'reta vortaro' is not a 'dictionary made of net', just an internet dictionary. The Slavic languages that Zamenhof was familiar with are very liberal, so to speak, in making adjectives out of nouns and then using these in all sorts of meanings. Still, perhaps the Esperanto community has come to use 'taso de kafo' instead of 'kafa taso'. That I don't know.
What is a difference between "taso de kafo" and "taso da kafo"? Is it like "cup for coffee" and "cup of coffee"?
If 'taso da kafo' means 'cupful of coffee', why is it accepted by Duo here where the cup is empty? Is it accepted in error?
How can a "taso de kafo" be a "cup for coffee", instead of a "cup of coffee". Wouldn't "cup for coffee" = "taso por kafo"?
Yes. There are other valid responses mentioned in the discussion above. But if you want to use it as a compound word, it must be either kaf-taso, kaftaso, or kafotaso. I can give you a more grammatical explanation if this is still a source of confusion.
I like kaftaso best, with kaf-taso being the same thing but stressing the compound, but why is "kafotaso" valid? That literally means "coffee-noun-cup-noun", and out loud could be taken as some talking really fast about his cup of coffee. Is it just because people are really used to saying words with their part-of-speech endings and dont like dropping them as roots?
No, the middle o of kafotaso is purely for reasons of euphony (nice sound) and pronounceability. English speakers don't mind an -ft- sequence, not even at the end of a word, but in many languages, that would be an odd and difficult cluster.
NOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! MI BEZONAS MIAN KAFON!!!!!!!!!!!!!!