Final answer:
The opening movement of Mozart's Symphony No. 40 is in sonata-allegro form, which consists of three main sections: the exposition, development, and recapitulation.
Step-by-step explanation:
The form of the opening movement of Mozart's Symphony No. 40 is sonata-allegro form. It is a common form used in classical symphonies and consists of three main sections: the exposition, development, and recapitulation. In the exposition, the main themes are introduced. The development section explores and develops these themes, often introducing new ideas. The recapitulation brings back the main themes and resolves the piece.