How do I know which case to use in German?

There are four cases in German:NominativeAccusativeDativeGenitiveNominate case defines the the subject in a sentence. The subject is the thing or person that does the action.E.g. The postman delivers the parcel. (Der Postbote bringt das Packet)Question: Who (or what) delivers the parcel? --> Answer: the postman. The postman is the sentence subject, or Nominative.
Accusative is equivalent to the English direct object. Generally, the Accusative case indicates the thing or person that "gets" the action.E.g. The postman delivers the parcel. (Der Postbote bringt das Packet)Question: What (or who) does the postman deliver? --> Answer: The parcel. The parcel is the Accusative case
Dative is equivalent to the English indirect object. Generally, the Dative case stands for the thing or person that receives the direct object (accusative case).E.g. The postman delivers the parcel to Ms Green. (Der Postbote bringt das Packet zu Frau Green)Question: (to) whom does the postman deliver the parcel? --> Answer: to Ms Green. Ms Green is the indirect object, or Dative case
Genitive case expresses belonging or possession. In English this is shown by an apostrophe 's.E.g. The postman's parcel. (Das Packet des Postbotens)Question: Whose parcel? --> Answer: the postman's (parcel).