Learn German: Derjenige, der den Beitrag schreibt, sieht keine Werbung. - The one m who writes the post sees no advertisement.

Derjenige specifically means the one or that person. It is a demonstrative pronoun that points directly forward to the relative clause (der den Beitrag schreibt). While you could technically say Der, der..., using Derjenige sounds much more elegant and strongly emphasizes that you are defining a specific person based on the action they are doing.

How do you know to use the masculine form Derjenige here?

It is masculine because we are making a general statement about an unspecified person, similar to saying he who in English. In German, the masculine singular form Derjenige is traditionally used as the default for a generic person. If we specifically meant the woman who, we would use Diejenige.

Why does the verb sieht come immediately after the comma?

The core of the main sentence is Derjenige sieht keine Werbung. The relative clause (der den Beitrag schreibt) simply interrupts it to describe the subject. Once the relative clause finishes and is closed off by the comma, the main sentence picks up exactly where it left off. Since the subject was already stated, the next word is the verb sieht.

Why is it den Beitrag instead of der Beitrag?