Learn German: Er stellt den Computer auf den Schreibtisch. - He is putting the computer on the desk.

auf is a "two-way preposition". It takes the accusative case when it describes a movement toward a destination (answering "where to?" or wohin?). Since he is moving the computer onto the desk, we use the masculine accusative form den. If the computer were already sitting on the desk with no movement involved (answering "where at?" or wo?), it would take the dative case: auf dem Schreibtisch.

Why use the verb stellen here for "putting"?

In German, you often have to be specific about how you put something down. stellen means to set something in an upright or standing position. A computer (like a monitor or a tower) is usually placed upright, so we use stellen. If he were laying something flat on the desk, like a piece of paper, he would use the verb legen.

Why is it den Computer and not der Computer?

Computer is a masculine noun (der Computer). In this sentence, it is the direct object—the thing that is being put on the desk. Because it is receiving the action, it takes the accusative case, which changes the masculine "the" from der to den.

How is the word Schreibtisch formed?