Use of Cases: Nominative, Accusative, Dative, and Genitive

Nominative Case

• The nominative case is the basic form of the noun, and it's used for the subject of the sentence. Basically, it covers the 'who' or 'what' that is doing the action.
• For example: "Der Junge spielt Fussball." (The boy plays football).

Accusative Case

• The accusative case is used for the direct object of the sentence – essentially, the 'who' or 'what' that is having the action done to it.
• For example: "Ich habe einen Apfel." (I have an apple).
• Keep in mind, in the accusative case, 'der' changes to 'den', and 'ein' changes to 'einen'.

Dative Case

• The dative case is used to identify the indirect object of a sentence, basically the receiver of the action performed.
• For example: "Ich gebe dem Mann das Buch." (I give the man the book).
• In dative, 'der' changes to 'dem', 'die' changes to 'der' and 'ein' changes to 'einem'. The plurals add an 'n' or 'en' unless they already end in 'n' or 's'.

Genitive Case

• The genitive case is used to show possession or belonging – 'of' or 's in English.
• For example: "Das ist des Mannes Hut." (That is the man's hat).
• The genitive case is becoming less common in everyday German speech, but is still very important in formal written German.
• In genitive, 'der' and 'das' change to 'des', 'die' changes to 'der' and 'ein' and 'eine' become 'eines' and 'einer' respectively.

Remember, the use of cases is a vital part of German sentence construction. It provides context to the roles different nouns play in a sentence. Practice frequently to become familiar with their usage.