The prelude is written in a classical ternary ABA form with exact recapitulation at the end. By the way, the final cadence of section A reminds very much the opening phrase of the choral "Jesu, meine Freude".
In section A (measure 1-6), composer establishes the tonic key of D minor and shows the first musical idea of the work. The texture varies between 4 and 5 parts (6 parts at the beginning of the 2nd measure) so anyone interested in learning it will do well to practice all 15 combinations of solo voices, 2-part and 3-part combinations before putting everything together.
The first episode in section B (measures 7-12) is written for 3 upper parts without the pedal. Here the composer explores a few compositional devices such as runs in the left hand, descending sequence and right hand suspentions. They lead to a cadence in F major (the relative key of D minor).
In the second half of section B (measures 13-24) we can see the ascending chromatic sequence which moves through various related keys, namely B flat major, C major, and D minor.
In fact, the entire episode is constructed from sequences because right after this ascending version, we can see two descending versions (in 4-part texture and in 3-part texture) which lead to the scalar passage connecting section B with recapitulation (measures 25-31).
The most common mistakes I see people make when practicing this prelude is playing it without paying too much attention to detail which results in a sloppy articulation because the texture might be too complicated to play all parts together right away.
Playing without forcing oneself to stop frequently and correct the mistakes a few times is another mistake which slows down the progress of an organist and even might result in unnecessary frustration.
I have prepared a score of this composition with complete fingering and pedaling written in which greatly facilitates practicing and learning process of this exciting work.
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