Broadly speaking, the first movement can be divided in to three pieces, each beginning with a version of what I have denoted A_0, the introduction to the piece inwhich motifs float around in a constant void, gaining energy, and morphing into musicalthemes. If we were to impose the sonata structure onto the piece, we would say that the three pieces are the exposition, development, and recapitulation. The exposition introduces two large thematic blocks separated by a transition that modulates this section from the tonic to the relative major.
The first thematic block is much easier to recognize as it is always preceded by A_0, the introductory phrase. Moreover, A_1 is the most easily recognizable theme inthe first movement; it emerges out of A_0 and is loudly played by the entire orchestra inunison. Although not all of A is homophonic, the melodic line is always clear throughoutthis section. In fact, Beethoven clearly wants to impress this theme in our memories. Not only does he make it loud and clear, he also repeats it, with slight modifications the second time.
Beethoven then modulates from the tonic to the relative major key. The modulation happens quickly and is extremely subtle, almost hidden by the many series of ascending sequences written into both the transition in part B of the exposition.Moreover, the transition and part B share many common characteristics, such as texture,tempo, and dynamic. Since the modulation happens so soon, perhaps the transition is meant to be very short and the five (or six) different themes following the modulation are the themes that compose part B. This makes sense time-wise as well, for a short transition will make parts A and B of the exposition approximately the same