"He does not eat red soup."
Translation:Er isst keine rote Suppe.
Quick rule that Duolingo taught me. Negative sentence with no noun or noun with definite article -> nicht; negative sentence where the noun has indefinite article or no article -> kein.
I've been told that germans will understand you if you get this wrong but it will sound like "foreinger's German" HTH
Correct. A helpful guide: http://german.about.com/od/grammar/a/German-Negation.htm
Earlier i had a sentence, "Mit einem grünen Kleid." Now here, they said in a discussion below that, einem provides the information of the gender/nature of Kleid and hence, Grün doesn't need to and by some rule, grünen will have an "en" ending. I tried to apply a similar law here and put, "Er isst keine rot Suppe" can anyone explain with Grammar rules, why is this wrong?
This is a problem with the previous advice being incomplete. The "-en" guide only applies to adjectiver endings that are: dative, genitive, masculine&accusative and all plurals while a different rule applies to the other combinations. Personally I don't find that any easier to remember than just learning the 3 adjective ending tables (and spotting the patterns in them). It is tedious but if you want to write German well you need to know it.
Luckily when speaking or hearing German the adjective endings are low down on the list of priorities.