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The best explanation is with an example:

Ich habe ihm gesagt, dass ....

"Er" becomes "ihm" in the dative.

If we have a "normal" relative pronoun, then we get

Der Mann, dem ich gesagt habe, dass ...

"Der" becomes "dem" in the dative. So far so good.

But what happens if we have a genitive relative, eg

Der Mann, ??? dessen Sohn ich gesagt habe, dass ...

We'd have to "decline" "dessen" in the dative, but is there such a thing? Are we then forced to put a "zu" in the ??? part? Or does the pronoun become "dessem" (I think not)? Or yet something else?

marked as duplicate by Carsten S, boaten, Ingmar, Em1, Loong Dec 19 '14 at 9:11

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The key here is that dessen isn't the word that needs to be declined. It's Sohn.

Now to explain why, we need to think about their roles.

  • dessen: depicts the genitive as a relative pronoun. This could just as easily be a noun in the Genitive case in a normal clause.

  • Sohn: the actual object of the relative clause, i.e. the person you actually told something.

Since Sohn is the person you actually told it to, it's the noun that should be in the Dative case. This is hidden in this sentence but would be visible if there was an adjective or if it was a plural form.

Der Mann, dessen ältestem Sohn ich gesagt habe, [...]

Der Mann, dessen Kindern ich gesagt habe, [...]

In short, you don't decline dessen because that's already a relative pronoun in its declined form, so to speak. You decline the actual object, but that won't be visible under certain circumstances.

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