I recently started playing piano.
While learning and performing new pieces, I found that it helps me a lot to have some certainty of what's going on with the notes, the chords, and the harmony.
I decided to learn these as my first 3 pieces: Beethoven's Bagatelle No. 25 in A minor and Piano Sonata No. 14 in C-sharp minor Op. 27 No. 2, and Chopin's Prelude Op. 28 No. 4.
I have no issues with the first two. I understand what's going on in the harmony, and roman numeral analysis was straight-forward. Chopin's prelude Op.28 No. 4, in the other hand, confuses me. I don't even know where to start in order to understand the function of each chord.
Here is the piece in question:
And the first measures:
It starts with i - V: Em to B7sus4 (i think?), and from there it goes through a series of subtle chromatic movements, involving only one note of each chord, slowly moving semitone by semitone. That's where I get lost. The functions of the chords stop being obvious to me.
How do you perform harmony and roman numeral analysis on works like this, with so much chromaticism going on? Where do you start? How is the function of the chords deduced/derived?