• A note is added to a diatonic major scale degree ^ann in the key in which we are at first; this note we hear at first as minor seventh; it seems that the major traid therefore is turned into a dominant seventh chord 
• When the chord resolves it turns out that the (perceived) minor seventh was no minor seventh after all, but an augmented sixth instead (or: it turns out that the seventh has changed to an augmented sixth). Which means that the chord turns out to be an Italian or German augmented ( #IVdv6 or #IVdv6/5 ) and to resolve to the dominant of the new key. In other words: We have made a modulation. 

• Measures 17-24 are a period (thus half as long as the period in 1-16); the antecedent ends with IAC, the consequent ends with PAC
• Measures 25-28 form the 'middle section' of the ternary, as they contrast with their surroundings, especially since they are all the time on the dominant
• The last 4 measures (29-32) are a varied repeat of 17-20, in which the close is changed in a PAC.

The movement is a large binary, though it seems at first sight rather a ternary:
 

1-29	A-section
1-9	main theme in D minor Periodical structure, slightly irregular: 5+4 measures. Antecedent ends on IV! (=plagal half cadence)
9-17	transition modulation from D minor to C major  It seems kind of problematic to label this section plainly, or only as transition: The character is quite 'thematic' The modulation ends in C major, whereas we 'should' end in F major or eventually A minor; only after measure 17 the 'correct key' A minor comes into view.
18-29	B-section
18-26	second theme in A minor Initially seems not really 'thematic'. Could also be a closing section, were it not that there is a clear closing section still to follow.
27-29	closing section or codetta in A minor
30-43	retransition This section, especially at the beginning, rather gives the impression of an independent middle section - and then we might think that we are in a ternary form! I believe we can see it as retransition, mainly because of the very overviewable harmony / modulation; in fact we modulate immediately back to the home key: From the F chord, VI in A minor and III in D minor, we reach the dominant of the home key of D minor through just a few chromatic steps.
44-65	A'-section
44-52	main theme modulater from D minor to Bb major The theme returns, varied, and becomes unstable from the sixth measure: the consequent modulates, and ends with deceptive cadence. Caused by this 'open end' the theme is now much stronger connected with the transition.
53-56	transition in Bb major The modulation has already taken place in the main theme; the transition confirms the key of B flat major, and he is much shorter than in measures 9-17: Of the original nine measures only the last four are quoted here.
	B-section
57-65	second theme in D minor Quotation (transposed to the home key) of measures 18-26
	closing section The closing section is not repeated. Instead, we move to a long:
65-87	Coda in D minor Consists of two sections: 65- 76 and 77-87. In measure 65 the beginning of the coda overlaps with the end of the second theme. Apart from the very complex harmony in the first section of the coda it is striking that from measure 72 the retransition is partly quoted.