1. The exposition: The first section of the sonata-allegro form, the exposition, introduces the main themes of the movement and sets up their relationship. The exposition is usually divided into two parts: the first theme area, which begins with the main theme in the tonic key; and the second theme area, which begins with the second theme in a related key.
2. The development: The development section is a section of free composition based on the themes introduced in the exposition. The themes may be presented in different keys, rhythms, and harmonies. The development section provides an opportunity for the composer to explore the musical ideas presented in the exposition and create a sense of dramatic tension.
3. The recapitulation: The recapitulation is the final section of the sonata-allegro form, and it is where the main themes of the movement are restated in the tonic key. The recapitulation is usually divided into two parts: the first theme area, which is similar to the first theme area in the exposition; and the second theme area, which is similar to the second theme area in the exposition but may end differently.
The sonata-allegro form is a flexible musical structure, and composers have used it in many different ways. Some composers have written sonata-allegro movements that are very simple, while others have written movements that are very complex. However, despite its flexibility, the sonata-allegro form remains one of the most important musical structures in classical music.