This tune is fairly simple in its chord options, but not simple in the fact that they are mostly barre chords! It’s written in the uncommon mode of G aeolian, whose key signature is the same as that of B♭ major. Therefore the available chords are G minor, A diminished, B♭ major, C minor, D minor, E♭ major and F major. The simple chords can be broken down into just I, IV and V aka Gm, Cm and Dm.
In the basic chords I have replaced the third and fourth bars, which are really just another chord I section, with B♭ major. This works because G minor’s related major is B♭ major (a minor chord’s related major chord is always two notes above it in the root scale) so the two can be used interchangeably to break up lengthy chord I sections.
This tune is somewhat unusual in that in the second half of the A part (second line) it doesn’t go to chord V on the seventh foot tap but on the fifth. This is clear from the melody notes in bars 13 – 16; “D – D C B♭ – A – G – – – G – – -”.
In the B part, the second line could really have been a huge chord I section with chord V marking the end of the section as usual. However, I have used an E♭ chord instead of the first chord I. You can always replace a minor chord with its related major (whose root note is two above it in the key scale; in this key B♭ major would replace G minor) or, for a more wistful sounding substitution, with the major whose root note is two below it in the root scale, as in this instance where G minor is replaced by E♭ major. I feel that this “wistful” sounding substitution fits particularly well with the long, high B♭ note played in this bar.
Bars 11 and 12 of the B part have been accompanied with chord V. This is not something I would commonly do, but I didn’t want to go from my E♭ straight back to chord I (if you add a substitution and then revert to the “standard” chord I it really feels like you’ve started a story and given up half way through) so as there was still plenty of chord I section available, it made more sense to resolve back to my chord I by going via chord V. If you have a long section of any chord you can always break it up by resolving to said chord from the chord whose root note is a fifth above it in the given key.
In this version of the chords, the A part is the same. The B part however has been changed to use a descending chord run going down the scale from Gm all the way to Dm (chord V). To keep the descending run going in bars 13 and 14, chord I has been replaced with chord IV (Cm). It is always acceptable to put chord IV or any of its substitute options in a chord I section.
The section ends in the usual way by resolving from chord V (Dm) back to chord I (Gm).
In this version of the A part I have replaced each of my chords with the jazzy tetrad shape applicable for the key of B aeolian. As this key has the same key signature and therefore chords as D ionian (major), my tetrad options are: B minor 7, C# ½ diminished, D major 7, E minor 7, F# minor 7, G major 7 and A (dominant) 7.
I have more or less done the same in the B part except that I have also reinstated the V chord for the 7th foot tap.