masculine, feminine, and neuter
how to differentiate them?
The most simple answer is: there is no way, you have to learn words together with their gender.
A little more complex: every language has some patterns and there are cases when you can tell the gender of the word. For example, all words that end in "-tion" or "-sion" in French are feminine. Words ending in "-ung" and "-heit" or "-keit" in German are feminine. Most words that end in "-а" or "-я" in Russian are feminine. However, there is no complete set of rules that would allow you to tell the gender of any word, so you'll have to memorize it.
I think this discussion should be moved to the German section, I don't know if an admin should do it.
Regarding how to understand genders... you can find here some tips... http://german.about.com/library/blgen_der.htm
The truth is that you need to learn the articles and the cases, and... always learn a word with its article, or in a context where you are sure of the gender of the word.
Der Hund Das Buch Die Wand Die Küche Das Ende Die Nummer Das Stück Das Zeug Der Schrank
Articles articles articles all the time.
I am studying German here on Duolingo especially to review genders, that I never remember. :D