How does Gilman use changes in language (diction, tone) to affect the narrator's shift from seeming sanity to madness?

We can distinctly see the changes in the narrator through the language she uses internally throughout the story. In the beginning, her thoughts are fluid, and her speech is controlled. As the story continues, we can envision her descent in madness through the distortions she describes in the wallpaper itself, and her fear of John and Jennie's intentions.

As I've already stated Gillman's use of syntax begins in a controlled way. The narrator is calm and well able to communicate lucid, complex thought. The sentences in this section are longer, they flow. In contrast, as the narrator becomes more and more erratic, her sentences become shorter and clipped. She seems incapable of conveying a complete thought, which quickly allows the reader to experience the deterioration of her mental condition.

In addition to what has already been said: In the beginning, we see the narrator begin every section where she writes about the wallpaper by telling us she IS now talking about the wallpaper. Later on, however, the wallpaper has become such an odsession that she only writes about patterns and colours changing without first stating that it is the wallpaper she is referring to, indicating that it is now occupying most of her thoughts, and this obsession is one thing that signals madness.