Similar to other Classical symphonies, the first movement of unfinished symphony adapted sonata form as the skeleton. Despite the use of symphonic sonata form, Schubert begins the piece by an 8-bar quiet solo Preface theme which is played by the cellos and double basses in unison. The low registers and softly played Preface theme creates a mysterious and mournful effect. Also, the theme ends with single long note F #. This gives an unfinished sense and some consider the Preface theme as part of the primary theme. It is unconventional as Classical symphonies usually start with a brief introduction or directly with the primary theme.
Followed by the Preface theme, the primary theme begins with the murmuring in the violins and stepwise motive from the Preface theme. The primary theme does not end in a normal way with a cadence in its home key. Although there is a cadence in its relative major, D major, it expands and link to the non-modulating bridge section which contributes to its structural instability. The perfect cadence in B minor appeared only at the end of the non-modulating bridge.
In addition, Unfinished Symphony lack of transition. There was no transition between the lyrical first theme and the second theme. He used a non-modulating bridge to connect the two themes. Instead of having transitory passage to modulate from tonic to its relative major, the bridge remained in the tonic b minor. Thus some even considered the bridge as part of the primary theme as it included a perfect cadence in its tonic. The real transition between the primary and secondary theme is abnormally short, just four measures and it just modulated to the submediant in the last measure. The secondary theme itself is similar to a miniature aba form as it consists of a short development