Final answer:
Theme 2a in the first movement of Eine kleine Nachtmusik contrasts with theme 1 through a change in rhythmic structure, transitioning from a strong and steady rhythm to a more delicate and syncopated series of notes, reflecting a different mood within the movement.
Step-by-step explanation:
In the first movement of Eine kleine Nachtmusik by Wolfgang Amadeus Mozart, theme 2a contrasts with theme 1 primarily in terms of its rhythmic structure and mood. While theme 1 is characterized by a lively and assertive rhythm that is typical of an opening theme in a classical sonata form, theme 2a introduces a change in mood.
The rhythm becomes more delicate and features a series of playful, syncopated notes, which is different from the straightforward and steady rhythm of theme 1. The contrasts between the two themes highlight the variety and complexity of the composition, showing Mozart's ability to create distinct musical characters within a single movement.