This is a sentence from children’s book I’m working through:
Mimili läuft in den Garten.
Garten is masculine dative here. So shouldn’t it be dem? Or is Garten plural here?
The plural of Garten ist Gärten. That means Garten in this sentence is in the accusative. In is a two-way preposition; since it’s in the accusative, that indicates movement. This sentence means:
Mimili runs into the garden.
No. The issue is with the assumption that Garten is in the dative.
Mimili läuft in dem Garten == "Mimili is running in the garden." (and is arguably not very correct German)
Mimili läuft in den Garten == "Mimili is running into the garden." (accusative)