stellen / stehen

stellen means to put something upright, like a bottle, a vase or a chair. It answers the question wohin? and takes an accusative: Ich stelle die Milch in den Kühlschrank. stehen means to be standing. It answers the question wo? and takes the dative: Die Milch steht im Kühlschrank. So stellen is the action, stehen the resulting position.

The choice of case follows the question: movement to a place takes the accusative, location takes the dative. But the verb itself depends on the final position of the thing. stellen is for upright things; flat things take legen and liegen instead. A common mistake is using stellen for something laid flat, like Ich stelle das Buch auf den Tisch when the book is lying down, or mixing up the cases, like Ich stelle die Milch in dem Kühlschrank. Note that aufstehen and vorstellen are different verbs and not part of this pattern.