How did Haydn use key contrast and themes in his sonata-form movements?

In the Classical period, Joseph Haydn utilized sonata form as a fundamental principle of musical organization, where key served as the primary articulator of structure.1

Haydn’s approach to this form was characterized by several distinct techniques:

• Monothematicism: Unlike composers who relied on strongly differentiated themes, Haydn often employed a "monistic" thematic approach, using the same basic theme or motive in both the tonic and the contrasting key areas.1
• Key Contrast: He effectively utilized dualistic key schemes to create dramatic tension and resolution. His work often involved wide-ranging modulations, and he frequently set slow movements in keys that were only distantly related to the tonic of the first movement.12
• Thematic Economy: Haydn’s themes were often reduced to motives or tags, where their harmonic implications—which define the sense of key—took precedence over their melodic attractiveness.1
• Structural Development: He was known for his adventurousness in development sections, where he would introduce new thematic material and strike out into remote keys, demonstrating a high level of formal craftsmanship.2