How does the exposition function in sonata form?

In sonata form, the exposition functions as the initial section where the primary musical material is introduced and a fundamental tonal conflict is established.12

Its primary role is to create a dynamic opposition of keys, rather than merely presenting a contrast of themes. The exposition typically begins in the tonic, or home key, and then moves to a closely related but distinct key—usually the dominant if the home key is major, or the relative major if the home key is minor. This tonal shift creates a sense of tension and opposition that the subsequent development and recapitulation sections must eventually resolve.12

While the exposition often features two or more distinct themes, the number of themes is of secondary structural importance compared to this tonal design. The thematic organization serves to help the listener navigate the movement's tonal landscape, though composers sometimes create additional interest by obscuring the relationship between thematic and tonal structures. Ultimately, the exposition sets the stage for the entire movement by defining the musical subjects and the tonal "home" from which the music departs and to which it must return.12