Sonata form is a rich and complex subject, often considered beyond the understanding of all except well schooled musicians. But some of the basics are pretty easy to understand and, more important, hear.
First, there are three main sections in the sonata form. There is the Exposition, in which two or more themes are presented, in different keys. The Exposition is often repeated.
Then comes the Development section, in which musical elements of the Exposition are “developed” by various means, including through sequential repetition and harmonic instability.
The Development leads to the Recapitulation, which is a restatement of the Exposition, now all “resolved” into the home key. Usually a Coda of some kind caps off a sonata form movement. You can read more about sonata form here.
My intention here is merely to give you the timings in the recording below of where these things happen in the first movement of Beethoven’s Symphony No. 1.
Before the Exposition begins, Beethoven gives us a slow introduction, a prelude.
The Exposition proper starts with the first theme at 1:20. The tempo is Allegro con brio.
The second theme arrives at 2:04.
The repeat of the Exposition begins at 3:14. It is a literal repeat of the entire Exposition, without the slow introduction.
The Development is launched at 5:10.
The Recapitulation happens at 6:32, with the second theme (now in the home key of C major) arriving at 7:05.
You will sense that the movement could end at 8:12, but at 8:13 Beethoven adds the Coda.
(After that, the recording includes the other three movements. Enjoy.)