The three basic forms are called: Exposition, development, and recapitulation.
2. What musical events occur in each section?
The main thematic substance for the movement is the Exposition: one or two melodies commonly called theme one and theme two. They are frequently in opposing styles and in differing keys. Due to the different keys they are usually interwoven with a modulation section, and may be repeated. Stereotypically the exposition ends with a final refrain, a codetta, or both. The Development Section is where the composer uses his inspired creativity to cultivate the themes stated in the Exposition. This is where the choral and textural possibilities of the thematic material are revealed.The composer may do many things to the themes by inverting, sequencing, or distorting the melody. The composer may also change the structure, key, tempo, or mix the melodies together. From my reading and research it seems to me there is not a lot of restriction on what a composer can do in this section. In the transition for the Recapitulation the piece will usually return to the tonic key unless there is material that has not been played in the tonic key it will be resolved by being played as a whole or in part in the tonic. The conclusion may be in a coda after the final rhythm of the recapitulation.
The Recapitulation section begins with the Composer returning to the Exposition and paraphrases the themes as they were first stated. I say paraphrases because the Recapitulation is not just a duplication of the Exposition. The keys and theme could be changed with some minimal development.
3. How does the structure of the sonata-allegro form relate to key relationships?
The Exposition begins usually in the Tonic Key but can sometimes puzzle the audience. Ok hold on now because this next part might get a little confusing. I will try to explain as best I can. Somehow this reminds me of the old who’s on first...