dem, denen, dessen, deren

When the relative pronoun is a dative object, it has these forms:

• dem for masculine and neuter nouns
• der for feminine nouns
• denen for plurals (not den!)

You need them after verbs that take the dative, such as helfen, danken, gehören, gefallen, vertrauen, schmecken, and for the person who receives something with geben, schenken, erklären: die Frau, der ich geholfen habe (the woman I helped).

The genitive forms mean whose: dessen for masculine and neuter, deren for feminine and plural. A noun without an article follows directly: mein Freund, dessen Vater in Wien lebt (my friend whose father lives in Vienna). The form depends on the owner, not on the thing owned.