Hey everybody ! I haven't exactly understood - when should I use Ein, eine and einen ? Thanks for helping (:
For feminine nouns: "eine" (nominative and accusative)
For neuter nouns: "ein" (nominative and accusative)
For masculine nouns: "ein" (nominative); "einen" (accusative)
"Ein/eine/einen" are so-called indefinite articles. In English, these words all mean "a" (or "an" before vowel sounds). E.g. "a man", "an appointment".
In German, the form of the indefinite article changes depending on the gender and case of the noun they refer to.
There are three grammatical genders in German: masculine, feminine and neuter. Each noun is either masculine, feminine or neuter.
Masculine and neuter nouns get the article "ein". E.g. "ein Mann" (a man, masculine); "ein Apfel" (an apple, masculine); "ein Haus" (a house; neuter); "ein Mädchen" (a girl, neuter).
Feminine nouns get the article "eine". E.g. "eine Frau" (a woman, feminine), "eine Frage" (a question; feminine); "eine Kartoffel" (a potato; feminine).
(As you have probably noticed, gender is assigned mostly arbitrarily and has to be learnt by heart when you learn the noun. You can also look it up in an (online) dictionary.)
These forms of the articles are called nominative or basic forms. They are used when the noun is the subject, i.e. the actor in a sentence. E.g. "Ein Mann liest das Buch" (A man is reading the book); "Eine Frau ist nett" (A woman is nice).
The nominative forms are also used after the verbs "sein" (to be) and "werden" (to become), e.g. "Er ist ein Mann" (He is a man); "Sie wird eine Frau" (She becomes a woman).
2) Case (= role of a noun in a sentence)
However, not all nouns are the subject (actor) in a sentence. Sometimes, they are at the receiving end of the action, e.g. "She hits a man". In this sentence, "a man" is not the subject, but the so-called direct object. For almost all direct objects, German uses the accusative case. (The accusative is also used after certain prepositions, e.g. after "ohne" (without)).
The good news is that for neuter and feminine nouns, the form of the indefinite article stays the same in the accusative case as in the nominative case that I mentioned above: "ein" (neuter; nominative and accusative) and "eine" (feminine; nominative and accusative). E.g. "Er sieht ein Haus" (He sees a house; neuter accusative); "Sie schlägt eine Frau" (She hits a woman; feminine accusative).
Only for masculine nouns, the article changes: "ein" (nominative) becomes "einen" (accusative). E.g. "Sie schlägt einen Mann" ("She hits a man"; masculine accusative), but: "Ein Mann ist nett" (A man is nice; masculine nominative).
Apart from the nominative and accusative, there are two other cases: the genitive and the dative. They, too, have special forms for the indefinite article. See: http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-Art/Art-Indef.html