The top staff (line) is for a (woman's) singing voice. The bottom (grand) staff is for a simple piano accompaniment part. The bass should alternate between 'F' and 'D' after each pair (or set of 4 ?) of measures. The harmonic structure could be analysed as alternating between 'F' Lydian mode ,and 'd' dorian mode . Stephen Stills actually used a D7 chord (which consists of a 'D' Major triad -- and that features the raised 3rd degree: "F#"). Sometimes that should be used, but I often prefer a 'd' minor (7) basis for these chords -- which instead uses the F_natural pitch. See "Suite: Lani Brown Eyes" -- which is a post to a recent thread.