Questions & Answers about Я хочу зайти в магазин вечером.
The choice depends on motion vs. location:
- в + accusative = motion to / into a place
- в магазин – into the store (you are going there)
- в + prepositional = location in a place
- в магазине – in the store (you are already there)
In Я хочу зайти в магазин вечером, the verb зайти expresses movement into the store, so Russian uses в + accusative → в магазин.