When there is a "movement" in sentence by a subject , then always use "Akkusativ". Zum Beispiel- Ich gehe in den Park( I am going to park - here there is an action by subject) . 2. When there is "no Action" by a subject, always use "Dativ" .Zum Beispiel- ich bin im Park( in+dem=im Park) . I hope that you get it. Danke schön 🌻🌻