Allegro ma non troppo, B major, 4/4 time

This movement, while adopting the framework of sonata form, attempts to expand it through modulation and the introduction of multiple themes.

At the beginning, the tonic triad of B major (the home key) is presented as an arpeggio in unison, and in the third measure, a dominant seventh of C-sharp minor suddenly appears. When this theme is presented again, after a general pause, it deviates to C major, and a new theme is presented in G major (m. 15). This second theme, in contrast to the staccato first theme, is played legato and flowing. The second theme modulates to E major in its last three measures, and a gentle third theme is presented in E major (m. 30). Up to this point, the key progression has been a descent by thirds: B major – G major – E major. This third theme modulates to F-sharp major, and in measure 42, a fourth theme appears, and the exposition closes in F-sharp major. Thus, while the exposition begins with a theme in B major and closes with a theme in the dominant key, maintaining the general framework of sonata form, it is noteworthy that two additional thematic areas are inserted in between. Later in his career, Schubert is known for breaking the tension between the tonic and dominant keys typical of conventional sonata form by inserting one mediant key area between them in the exposition (for details, see Piano Sonata No. 21, D 960). This very characteristic can be observed in the sonata form of this movement.

The development section is divided into two parts. The first half, by repeating motives from the first theme, modulates through mediant relationships from B minor to D major, F major, and A-flat major. Then, after passing through E-flat major, the second half, derived from the second theme, appears in B major. The return of the tonic key in the latter half of the development section, without waiting for the recapitulation, is a rare case in conventional sonata form, but there is a reason for this. In this movement, as already attempted in Piano Sonatas No. 2, D 279; No. 3, D 459; and No. 4, D 537, the recapitulation appears in the subdominant key of E major. Then, while maintaining the key relationships from the exposition, the second theme is recapitulated in C major, the third theme in A major, and the fourth theme returns in the tonic key of B major, bringing the movement to a close.