"How much soup do you have?"
Translation:Kiom da supo vi havas?
why isn't "supon" and is "supo"?, if you say "mi havas supo" it has a mistake because you should say "mi havas supon"
Because of the preposition - only in a few exceptional cases related to direction and motion will make a noun or adjective decline with the accusative after a preposition.
I think so, but I think it has a different connotation. Like in English, directly translated the sentence is saying "You have HOW MUCH soup?" there is definitely a different tone in that sentence, I'm not sure if the same rules apply to Esperanto, but just to be certain I would avoid writing it that way.
Forgive me for the noob question, I haven't been on Duolingo in like a week. Why is the word "da" used here?
Would "Kiom da supo havas vin?" be correct? Would I get rid of the accusative?
I don't believe that "vin" would be correct here, as that would signify "you" as the object that the soup has. "Kiom da supo vi havas?" or "Kiom da supo havas vi" or even "Vi havas kiom da supo", as mentioned above, all should be correct/acceptable. But "supo" is not given the -n marker, as nothing is given the accusative after "da/de" (See Tips & Notes: Da/De)