How do I know which case to use?

The case is determined by:

Verb: Many verbs require a specific case (e.g., sehen takes accusative, helfen takes dative).

Preposition: Prepositions are almost always followed by a specific case (e.g., für always takes accusative, mit always takes dative).

Sentence Function: Subject is always nominative, direct object is accusative, indirect object is dative, possession is genitive.