Hi, could someone please help me out with analysing the chords to this song. It's in the key of G, but there's some non - diatonic chords which I can't figure out where they come from. Basically I was wondering what techiques are being used, like modal interchange (if any), chord substitutions (if any), ii-V's etc.

A Section

G / Bb7 / Eb / Ab /
A-7 / D7 / G / D-7 G7 /

B Section

C-7 / F7/ Bb / Eb /
A-7 / D7 / G / A-7 D7/

C Section

G / Bb7 / Eb / Ab /
A-7 / D7 / G / D-7 G7 /

D Section

C / A-7 D7 / B-7 E-7 / A7#11 Bbdim7 /
A-7 / D7 / G / Rest /

There's a lot of circle of 5ths and V - I's going on, to be sure. I'd start there. I'm not sure what you know, but you should start on a piece that you feel like you can do, or learn about Modal Interchange, the key's are changing that's why this is throwing you. Follow the key changes, by Identifying the V- I - regardless if they are major or minor - there's a hint right there.