Dativ plural: den oder denen?
I've always learned that the plural form for dativ is "den". But sometimes i see that it's "denen". For example: Kennst du die Kinder, mit ..... er spricht?
My answer is "Kennst du die Kinder, mit den er spricht?"
But the correct answer is "Kennst du die Kinder, mit denen er spricht?"

Question

Dativ plural: den oder denen?
I've always learned that the plural form for dativ is "den". But sometimes i see that it's "denen". For example: Kennst du die Kinder, mit ..... er spricht?
My answer is  "Kennst du die Kinder, mit den er spricht?"
But the correct answer is "Kennst du die Kinder, mit denen er spricht?"

Answer

Correct is: ...Kinder, mit denen er spricht:
'denen' is the relative pronoun, 'den' the article.
You can see where it comes from: Er spricht mit DEN KinderN.
The dative plural is expressed by the 'den + n', so the relative pronoun must have the same.

Answer

"Den" is the plural dative form of the definitive article der/die/das, "denen" is the form of the demonstrative pronoun der/die/das (i.e., a replacement for the noun itself). Many forms are identical, but some are different. So it's "Er spricht mit den Kindern", but "Er spricht mit denen".

http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-Art/Art-Def.html
http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-der-die-das.html