How did Schubert expand the number of themes and tonal centers in sonata-form expositions?

Franz Schubert expanded the traditional sonata-form exposition by increasing the number of tonal centers and, at times, the number of basic thematic groups from two to three.1

While the Classical sonata form typically relied on a dualistic key scheme—often contrasting the tonic and dominant—Schubert’s approach in his later instrumental works modified this structure substantially. By incorporating three distinct tonal centers and thematic groups, Schubert moved away from the strict Classical reliance on a two-part conflict. This expansion had a significant influence on the subsequent Austro-German symphonic tradition, serving as a direct precursor to the expansive sonata structures later utilized by composers such as Anton Bruckner and Gustav Mahler.1