Molto moderato, B-flat major, 4/4 time

The exposition of this movement, written in sonata form, features themes in three keys, which has been called a "major contribution" to the history of sonata form. At the beginning, a lyrical first theme is played in B-flat major. This theme closes with a half cadence, and a softly played bass trill adds an ominous nuance (m. 8). When the subsequent statement of the theme closes with a perfect cadence, the theme reappears in G-flat major, supported by a new sixteenth-note accompaniment figure (m. 20). The theme returns in the tonic key, supported by a triplet accompaniment, and following a transitional section, the second theme is presented in F-sharp minor (m. 48). If enharmonic reinterpretation is not applied, the second theme is written in G-flat minor, revealing a mediant relationship with the tonic key. The second theme consists of two parts: the first part has a triplet accompaniment in the inner voices, with the outer voices carrying the melody. In the second part, the left hand provides an accompaniment of sixteenth-note arpeggios, while the right hand carries the melody. In this latter part, through further enharmonic reinterpretation, an anticipation of F major builds from m. 70 (F major triad in second inversion and F major dominant chord). Prepared by these, the third theme is presented in F major at m. 80. The codetta continues in F major (m. 99), and the exposition concludes.

Thus, in the exposition of this movement, the first theme is in B-flat major, while the second theme appears in F-sharp minor (mediant key), and the third theme in F major (dominant key). A typical sonata form can be dialectically interpreted as creating tonal tension (thesis and antithesis) by presenting two themes a fifth apart, which is then sublated (synthesis) when both themes return in the tonic key in the recapitulation. In contrast, in this piece, a theme in a mediant key is inserted between the tonic and dominant areas, thereby relaxing the tension that would otherwise be aimed for in the exposition. This clearly demonstrates Schubert's compositional philosophy, which emphasizes transition and change, differing from Beethoven's approach to musical construction marked by tension.

Development

The development section opens with the first theme appearing in C-sharp minor, and the third theme modulates sequentially from A major. A new theme introduced in D-flat major at m. 151 also modulates sequentially. When the bass trill appears at m. 186, fragments of the first theme's melody emerge, anticipating the recapitulation. Then, led by the characteristic trill, the recapitulation begins at m. 216.

Recapitulation

Two points regarding tonality are noteworthy in the recapitulation:

• First, the section that deviated to G-flat major in the middle of the first theme in the exposition modulates from G-flat major through F-sharp minor to A major (three sharps) in the recapitulation.
• Second, while the first and third themes return in the tonic key, the second theme is recapitulated in B minor (two sharps).

In both instances, the modulation is to a key with sharps that is neither a closely related key nor a mediant key, meaning it moves far away on the circle of fifths from the tonic key (which has two flats). These two points indicate that in this piece, an attempt is made to shift to keys even more distant than those in the exposition, within a recapitulation that typically aims to remain in the tonic key. This can also be seen as a result of Schubert's unique development of the sonata form, which was common at the time.