The exposition is like the first act of a play: we are introduced to the cast. Here the primary theme emerges (in the main key), then through a transition we arrive at the secondary theme (in a contrasting key), and it all concludes with a closing theme.
In the Classical era, the exposition was often repeated — giving the listener a chance to “think again” about the themes. Beethoven gradually abandoned mandatory repeats; Schubert and Schumann introduced a more lyrical and expansive character; and Mahler stretched them into 15-to-20-minute monumental musical tableaux.
