Voted Best Answer
Jul 06, 2020 - 09:00 AM
The reason is simple: after the preposition "mit" you always need the dative case. It shows up in level 2, session 12.
When I was still learning I needed to remind myself which preps used the dative case a lot, so I kept repeating the essential ones in the same order, here it goes (maybe it will help you as well ):
Aus Bei Mit Nach Seit Von Zu ALWAYS imply the dative case :)
Keep it up