In which section of a sonata form does the primary theme first appear?

In sonata form, the primary theme first appears in the exposition section. This section is crucial as it introduces the main musical ideas or themes that will be developed further throughout the piece. The exposition typically consists of two contrasting themes, often in different keys, which set the stage for the development that follows. The clarity of this structure allows composers to establish thematic material that can be manipulated and transformed in the development section, before returning to those themes in the recapitulation. The coda serves as a concluding section that brings the piece to a close, often summarizing or elaborating on the themes presented earlier.