How did sonata form use themes and changes of key to create structure?

In sonata form, structure is created through the dynamic interplay of tonality and themes, organized into three primary sections: the exposition, development, and recapitulation.1

Tonal and Thematic Organization

• Exposition: The movement begins in the tonic (home) key. It introduces contrasting musical subjects and moves to a new, closely related key—typically the dominant for major keys or the relative major for minor keys. This tonal opposition establishes the conflict that drives the movement.2
• Development: This section is characterized by tonal flux, where the music modulates through various keys. Themes are broken down, recombined, or transformed, creating tension and dialectical conflict.31
• Recapitulation: The movement returns to the tonic key, resolving the earlier tonal conflict. While the original themes are restated, they are now typically presented within the tonic key, providing a sense of synthesis and equilibrium.145

While thematic distribution helps listeners track the tonal design, the structural foundation of sonata form is the movement away from and return to the home key.124