The structure of the sonata is unconventional in that the piece opens with a relatively slow movement in the format of theme and variations. In contrast, the remaining movements of the sonata proceed according to classical principles in fast-slow-fast alternation. The third movement of this sonata incorporates a funeral march, clearly anticipating the watershed of the Eroica Symphony that Beethoven wrote the following year. This is the only movement from his sonatas that Beethoven arranged for orchestra, and was played during Beethoven's own funeral march in 1827.
This sonata is also unusual in that none of its four movements is in sonata-allegro form.
In most of Beethoven's four-movement sonatas, the third movement is in 3/4 and in ternary form, while the second movement is slow and in a different key from the other movements. In this sonata, the second and third movements have switched roles, where the second movement is the ternary scherzo and trio, while the third movement is the slow movement in the tonic minor.
The main theme of Schubert's Impromptu in A-flat major, Op. 142 No. 2 is strikingly similar to the theme in the first movement of Beethoven's sonata. The four-bar phrases that open these pieces are almost identical in most musical aspects: key, harmony, voicing, register, and basic as well as harmonic rhythm. Another less immediate connection exists with the main theme, also in A-flat major, of the Adagio movement in Schubert's piano sonata in C minor, D. 958. Indeed, Schubert may have borrowed these themes from Beethoven, as he often did in his compositions.
This sonata was greatly admired by Chopin, who repeated its basic sequence of scherzo, funeral march with trio, and animated, resolving finale, in his own piano sonata in B-flat minor. His first movement, however, is also animated and in sonata form, unlike Beethoven's Andante con variazioni.
^ see Wayne C. Petty's article, Chopin and the ghost of Beethoven, 19-Century Music XXII/3, 1999, pp. 281-299; see also Rosen, p. 150.
Rosen, Charles, Beethoven's Piano Sonatas - A Short Companion, 2002, Yale University Press, pp. 150-152.
Adolph Bernhard Marx,"Introduction to the Interpretation of Beethoven Piano Works", pp.110-113. IMSLP Books.