Questions & Answers about Ich stelle den Stuhl nicht neben das Regal.
Why is it das Regal here instead of dem Regal?
neben (next to) is a two-way preposition. This means it can take either the Dative or the Accusative case. Because you are placing the chair somewhere (a movement to a destination, answering the question "Wohin?"), you use the Accusative case. Since das Regal is neuter, its Accusative form is simply das Regal. If the chair were already sitting there, you would use the Dative: neben dem Regal.
Why do we use stelle instead of stehe for "put"?
In German, you have to be specific about how you place an object. stellen means to put or place something in an upright position (like a chair). stehen means "to stand" (the state of already being upright). Since you are doing the action of placing the chair, you use stellen.
Why is nicht placed right before neben das Regal?
When you want to negate a specific part of a sentence, you put nicht right in front of it. Here, you are specifically negating the destination: you might put the chair somewhere else, but not next to the shelf. With destinations and two-way prepositional phrases, nicht almost always goes right before the preposition.
Why is it den Stuhl and not der Stuhl?