Need some help with theory. This lovely piece in C minor moves in and out of G, Fm, D7, Bb, and so on. Is this an example of temporary keys and/or use of melodic minor or harmonic minor scales which are closely related? Does anyone have a music theory source that explains this? I have loads of theory books but most address chord progressions within the key (diatonic). I need help with the changes from minor to major that are not diatonic, particularly addressing the use of the minor scales mentioned above.

Thank you, Garbo

I assume you are referring to chords rather than keys? I'm not familiar with the piece at all but I'd guess at:

G: dominant (V) chord - common in minor at cadences (will often see melodic minor with this chord)
Fm: subdominant (iv) - normal in natural minor
D7: V7/V? ie part of a cadence in the dominant key (so yes a temporary key or tonicization)
Bb: VII (subtonic?) chord - normal in natural minor, often leads to the relative major (Eb in this case)

If you're referring to the keys of G, Fm, etc then their function/concept is essentially an expanded version of the above. That's my understanding anyway - could be wrong.

Hope that helps somewhat.

It's just modulating as most pieces do through a series of keys and then comes home to end. Doesn't seem too mysterious to me. Played well, very effective piece.