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A sonata form is a piece of music that has three parts in an ABA pattern. While Classical composers such as Haydn and Mozart used sonata forms in their work, the term was not applied to the specific pattern until later. Yet, understanding the sonata form is critical to understanding the symphonies and other large-scale works of music during the Classical and later periods of music. The main three parts of the sonata form are as follows:
Exposition: The exposition is the first part of the sonata form, and it is where the composer will bring in, or “expose,” all of the musical ideas that will be used in the piece. The first tune and key of the piece are used. The themes or subjects of the piece are also introduced; you may find it helpful to think of them as characters in a book. If you listen closely, you may be able to pick out these themes from the beginning of a sonata form. The two themes are known as the First Subject Group and the Second Subject Group. They may have a transition occurring between them.
Development: Development is the second part of the sonata form, and it is where the composer builds on the two themes from the exposition. Music scholars have sometimes described this as “fantasia” because the composer can run wild with the themes set up and create an infinite number of different variations. This is the part of the piece where tension and drama often build; it may create some discomfort in the music and it is not unusual for the listener to want to return to the original themes of the exposition.
Recapitulation: Recapitulation is the third part of the sonata form, and it is where the exposition is repeated (although it may be changed slightly). The tension and drama of the development part leaves and the piece comes back to a lighter, happier sound. Toward the end of the recapitulation, the piece returns to the original key that the exposition established as the main key.
Symphony

Allegro or opening sonata: This movement is often brisk