Prepositions preceding dative
Nouns following these prepositions are always in the dative case: aus, bei, mit, nach, seit, von, zu. Example: Ich gehe zu der Schule.
My high school German teacher had us memorize short lists of prepositions that use accusative and dative cases. Your post reminded me of that.
Dative: aus, ausser, bei, mit, nach, seit, von, zu
Accusative: durch, für, gegen, ohne, um
You're right, "mit" takes the dative. "der Benutzer" (sing., nominative), "dem Benutzer" (sing., dative) BUT: "die Benutzer" (plural, nominative), and, as in the test question: "deN BenutzerN" (plural, dative)
In one of the test questions, Duolingo says that it should be "Er spielt mit DEN Benutzern"... but Benutzern is masculine, so shouldn't it be DEM? I also learned that mit always takes a dative.
I'm afraid these are not prepositions but VERBS that follow prepositions define what case should be used. Compare 'auf dem Tisch stehen ' and 'auf den Tisch legen'. First verb means static position, second - the direction of movement. You can say also both 'auf der Straße gehen' and 'auf die Straße gehen; depending whether you are walking already by the street or heading to it.
And I find it strange that such an important topic does not seem to be discussed in this lesson.