When do I use Kein or Keine or Keiner? I don't understand the differences?
There is a section on negatives that will talk about using kein vs nicht: http://duolingo.com/#/skill/de/Negatives . The difference between kein, keine, or keiner will depend upon the case of the noun that it proceeds, and where it falls in the sentence (accusative case, dative case, etc.)
Kein is used for masculin and neutrum nouns (der or das); Keine is used for feminine nouns (der) Keiner is related to the Dativ case (only feminin nouns)
Kein is associated to the noun (it is a negation of the noun). For example: Ich habe keine Zeit (I don't have time). First, Zeit in German is a feminin noum (die) and because of that we use KEINE and not kein.
Nicht is associated to the verb (it is a negation of the verb) For example: Ich arbeite nicht. (I don't work)