There are four grammatical "cases" in German:
- Nominative, Accusative, Dative and Genitive.
- (Nominativ, Akkusativ, Dativ und Genitiv).
A lot of people don't realise, but English has "cases" as well, but only three:
- Subjective, Objective and Possessive.
Subjective = Nominative
Objective = Accusative
Possessive = Genitive
There is no "Dative" case in English.
These cases are basically the "roles" that words have in a sentence.
A word in the nominative case is the subject; in the accusative case: the direct object; in the dative case: the indirect object; and in the genitive case: the possessive object.
In this question:
- "Fisch" is the thing having something done to it; "being eaten", so it is the direct object of the sentence.
- In German, that means it is in the accusative case.
- "Fisch" is a masculine noun.
Masculine nouns in the accusative case have the articles „den/einen“ and their adjectives have an -en ending.
- „Der kleine Fisch schwimmt.“ - "The small fish swims" or "The small fish is swimming".
(Fisch is in the nominative case; it is the subject.)
- „Ich esse einen kleinen Fisch.“ - "I eat a small fish" or "I am eating a small fish".
(Fisch is in the accusative case; it is the direct object.)
- „Das Kind gibt dem kleinen Fisch etwas zu fressen.“ - "The child gives the small fish something to eat" or "The child is giving the small fish something to eat."
(Fisch is in the dative case; it is the indirect object.)
- „Das ist das Meer des kleinen Fisches“ - "That is the little fish's sea" or "That is the sea of the little fish."
(Fisch is in the genitive case; it is the possessive object)
Some masculine nouns are "weak", and the noun itself will have an -n added to it in every case except the nominative. A weak noun is one like „Junge“. (There's no way to predict what words are weak nouns, you just have to learn them.)
- „Ich schlage den Jungen“ - "I hit the boy" or "I am hitting the boy".
(Junge is in the accusative case, it is the direct object)
„Ich gebe dem Jungen etwas Geld“ - "I give the boy some money" or "I am giving the boy some money".
(Junge is in the dative case, it is the indirect object)
Doesn't that make it difficult to discern whether the noun is plural or not? Nope! If the noun is plural, it will use the article "die" in the nominative and accusative cases, and "den" in the dative:
- „Ich schlage die Jungen“ - "I hit the boys" or "I am hitting the boys".
„Ich gebe den Jungen etwas Geld“ - "I give the boys some money" or "I am giving the boys some money".
I haven't covered the other genders or plurals in the other cases, but:
- If you want to learn more about what articles or adjective endings to use in what case, go here: Wikipedia - German Declension
- If you want to learn more about what the cases are, go here: German.about.com - The Four German Cases
- If you want to learn more about masculine weak nouns, go here: Germanforenglishspeakers - Weak Nouns