What is the function of this chord progression in Schubert's "Winterreise"?

Question

I'm having a hard time figuring out how the chord progression from Schubert's Winterreise (D911), No. 11 "Frühlingstraum", measures 15–26 works (from the section marked "Schnell" to the section marked "Langsam".

The passage ends in A minor, suggesting the tonality of A minor overall. This can also be heard in recordings of the piece, where the ending on Am sounds very definitive and conclusive.

But I have a hard time figuring out the functions of the many chords. There are a lot of inverted French augmented 6th chords, but how they relate to the bigger picture, and lead towards the A minor, is hard for me to tell. So the general movement towards the section resolving on the A minor before the next part starts at "Langsam" is very hard for me to analyse.

Answer 1

There's a lot going on in this excerpt. Here's a general answer, with some pointers to get more information if needed.

“Top level” analysis

In broad terms, the passage is in A minor and breaks down in this way:

Mm. 15–16: E minor = v
Mm. 17–20: D minor = iv
  [Mm. 19–20: G minor = iv/iv]
Mm. 21–26: A minor = i

"iv/iv" is an applied chord, meaning "the iv chord relative to the iv chord". This is especially common in the case of applied dominant chords. See Confused on marking figured bass for modulating sequence for more.

Mm. 15–16: E minor (v)

• Measure 15 starts on a first inversion E minor triad.
• The second half of m. 15 is a ii chord in E minor (i.e., F#o), with an E pedal tone.
• Measure 16 returns to the first inversion E minor triad.
• The second chord in m. 16 is a common-tone Fr6, which then returns to the E minor triad.

Common-tone augmented sixth chords are a technique to expand a particular harmony -- in this case, E minor. They function in the same manner as common-tone diminished chords. For more, see A chord progression from Leavitt: how to analyze it correctly.

Mm. 17–20: D minor

• The E minor triad in m. 17 is a pivot chord. It’s still the v chord, but also serves as the ii chord in D minor, kicking off a ii-V progression leading to that key in the next measure. Note that the ii in this case is borrowed from D major, though modal mixture, as it would otherwise need a Bb rather than B natural.
• Measure 18 is D minor, with a common-tone Fr6 connecting the two instances of that chord.
• Measures 19 and 20 are iv/iv, which is to say, G minor. Measure 19 contains an Adim chord with a G pedal tone, mirroring measure 15. Measure 20 contains a common-tone Fr6.
• The D minor chord at the end of measure 20 rounds out the entire passage as primarily expanding D minor.

Mm. 21–26: A minor

• A minor arrives in m. 21 as a plagal cadence from m. 20, but is then confirmed with a solid V-i cadence moving into m. 22.

From measure 22, this entire segment occurs over an A pedal tone. The chords are analyzed according to the pitches above the pedal.

• Measure 22 confirms A minor via a common-tone Fr6.
• Measure 23 is a Neapolitan Six chord (over the A pedal)
• Measure 24 is again A minor, expanded by a common-tone Fr6
• Measures 25 and 26 are a V-i cadence in A minor.

For more on the Neapolitan chord, see What is a Neapolitan 6th?