How did the first movement of Beethoven’s ‘Eroica’ rework the formal conventions of the classical symphony?

Though composed within a sonata form (exposition-development-recapitulation-coda) analogous to the first movements of Haydn and Mozart symphonies, the first movement of the ‘Eroica’ is unusually complex for a classical symphony in structural terms. Unlike the length favoured by Haydn (the first movement of his Symphony No.95, for example, is 166 bars long) and Mozart (the first movement of the ‘Jupiter’ Symphony is 308 bars long), the ‘Eroica’ is almost 700 bars long and thus involves two crucial structural innovations. First, there are three subject groups rather than the standard two. The first subject is stated three times, twice in the tonic and the final time modulating to the dominant, and the second and third subjects cadence in the dominant (bars 83 and 144) and furnish important material for the development section (for instance bars 220-248 and 248-283 respectively), demonstrating how Beethoven used extra thematic material to account for the movement’s large dimensions.

Second, though a coda section in classical sonata form typically involved little more than a brief episode reaffirming the tonic key, the ‘Eroica’ first movement includes a coda lasting from bars 551-691 to offset the modulatory drama of the development section with tonic stasis for the closing quarter of the movement. Beethoven therefore reworked the standard proportions of material both within each section of the movement and across the movement as a whole to accommodate classical sonata writing within an expanded version of the form.