How does the development section change themes from the exposition?

In sonata form, the development section functions as a space of tonal and thematic exploration, where the material introduced in the exposition is treated dialectically. Rather than simply restating themes, the development section transforms them through several key methods:12

• Fragmentation and Recombination: Composers often break principal themes into smaller elements, recombining them to form new melodies or placing them into new contrapuntal relationships.1
• Tonal Flux: The section is characterized by frequent modulation, moving through keys that are often distantly related to those established in the exposition. Themes or fragments are frequently presented in these new keys.13
• Thematic Expansion: Composers may take a seemingly minor feature from the exposition and develop it extensively to reveal its hidden importance.1
• Introduction of New Material: Occasionally, entirely new themes are introduced to provide contrast or expand the scope of the movement, though some themes that appear new are actually decorated versions of motives heard earlier.1

Ultimately, these techniques serve to create a sense of instability that heightens the dramatic impact of the eventual return to the tonic in the recapitulation.13