My question is why does the F#m work as a preparation?
Although the right-hand has an apparent F#m chord, when the left-hand is included, the chord is revealed as a D# half-diminished chord. D#m7b5 (half diminished) is the ii chord in the key of C# minor. It's followed in the next measure (m. 10) by G# major, which, of course, is the V chord. So, measures 9-...
I doesn't modulate to c minor at all. It just cuts the scene and starts up in the new key. There IS, however, a pattern to the harmonic motion.
Throughout this song, the composer frequently moves to the relative minor of the tonic major of the current minor key. (Phew!)
Before your head explodes trying to unpack that last sentence, let me just show it:
The overall analysis goes like this:
measure 19 20 21 22 23 24 25 26 27
chord Bbm Gb Db Ab Cb Abm Bbsus Bb Cm
Roman numeral Bbm: i VI III VII bII
Eb : bVI iv V V vi
Although the modulation is to C minor, it helps to consider it in terms of Eb major. That clarifies the Bb chord ...
Let me start with the first question: all those chords in bars 9 and 10 iv -> ii -> V7 -> i cadence in c# minor. This cadence adds the sharps that would change e minor to E Major, so it feels like a lift to E major, landing on its relative minor, c#.
There are some similarities here. In bar 9-11 we moved from e-minor to c#-minor, down a ...