The Sonata No. 1 in F minor, Op. 120, No. 1 consists of the following movements:
Allegro appassionato, in F minor, in 3/4 time
The first movement is in sonata form. It begins with a solo piano introduction in three parallel octaves, outlining a recurring motif throughout the movement. The clarinet then enters with the slurred first theme. The piano takes over the theme, with the clarinet playing more of an embellishing role. It was normal in clarinet music before these sonatas for the soloist to play mostly, if not always, the melody. Brahms did not reduce the scope of the piano part to accommodate for the clarinet, but created a more equal and harmonious relationship between soloist and accompanist. [citation needed]. The quiet transition between the first and second theme is in D-flat major and features staggered entrances between the hands of the piano. The second theme introduces dotted rhythms and is marked marcato, contrasting with the first theme. It passes through many key areas quickly before finally resting on C minor.
The development begins by expanding on ideas heard in the introduction and transition. The piano plays with staggered hand entrances and joins the clarinet in recalling the second bar of the introduction. The music makes a false movement towards A-flat major, instead landing on E major, a tritone away. The introduction material takes over and winds down to pp. A subito forte evokes the second theme combined with staggered entrances from both piano hands and clarinet. The second theme is finally presented and leads to the recapitulation.
The introduction is restated forte in the key of C-sharp minor. Brahms brings the key back around to F minor and the first theme, transition, and second theme are heard again. Tonally, this section does not stray far from F although the music goes through major and minor sections. A final statement of the first theme leads into the coda, marked Sostenuto ed espressivo. The coda is slower in tempo and based on material from the introduction. The movement ends quietly in F major.