Supposing you know how to use in/into correctly in English, German shows you the way. In + Dativ is always "in", In + Akkusativ is always "into". Next question: when do I use Dativ or Akkusativ after "in" in German. One of the ways to describe the difference is to see if the situation is "dynamic" or "non-dynamic". In "Ich gehe in einem Park spazieren" you can think of the park as a box. You will be moving, but always inside the box. >> dative >> "in". Using the image of a box: "Ich stelle das Buch in den Schrank". the book is moved from one box (your hand/the table) into another box (the bookcase) >> Akkusativ >> into. So going from one box into the other is "into"
'Into' is accusative and 'in' is dative case.
Ich gehe im Park = I am walking in the park (dative).
Ich gehe in den Park = I am walking into the park (accusative).
In this instance, accusative indicates change (I wasn't in the park but I am going to be) and dative means you're walking in the park already and will continue doing so.
Ich gehe ins Klo (accusative) = I am going to the toilet
Ich gehe im Klo (dative) = I am walking around in the toilet
It is a bit difficult to get ones head around at first but it becomes second nature.
Dative Masculine = Dem | Neutral = Dem | Feminine = Der | Plural = Den
Dative Masculine = Den | Neutral = Das | Feminine = Die | Plural = Die