Explaination about why and when to use Eine/Ein/einen etc..
Explaination about why and when to use eine/ein/einen etc.. den/dem istead of der etc..
This would be better posted in the "German" subtopic, and not "Duolingo".
Read this wikipedia page or look up "german articles" elsewhere... If you don't know what German cases are, you will most likely have to read up on those too to get a better handle on the articles. http://en.wikipedia.org/wiki/German_articles