There are definite articles in the German language which can be used before nouns. A noun has a gender (masculine, feminine or neutral), a number (singular or plural) and a case (Nominative, Accusative, Dative or Genitive). These three words(Der, Die, Das) in you question are definite articles. When you are thinking which one to use you should consider number, gender and case of the noun it's referring to. For nominative case: der - masculine(singular) das - neutral(singular) die - feminine(singular) die -plural (gender doesn't matter) Here is a simple example. Suppose you want to say that "the car is expensive". "Car" can be translated as "Auto". Auto is neutral noun, here it's singular and used in nominative case. So you say "Das Auto is teuer". More examples: 1. Die Frau ist schön. The woman is beautiful. I use "die" because Frau is a feminine noun, singular, in nominative case. 2. Das Mädchen ist schön. The girl is beautiful. I use "das" because Mädchen is a neutral noun, singular, in nominative case. 3. Der Mann ist stark. The man is here. I use "der" because Mann is a neutral noun, singular, in nominative case. 4. Die Frauen sind nicht hier. The women are not here. Frauen - plural, nominative case

Just wanna add something to Zurab: "3. Der Mann ist stark. The man is here." is actually a typing mistake, "Der Mann ist stark" means "the man is strong".

Canoo.net and mein-deutschbuch.de are helpful. http://www.canoo.net/services/OnlineGrammar/Wort/Nomen/Genus/index.html http://www.mein-deutschbuch.de/lernen.php?menu_id=59 http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-dieser-jener3.html?MenuId=Word4241 http://www.mein-deutschbuch.de/lernen.php?menu_id=62