hi this is santy
for particular song to find out chords

i know that 1st we have to find out the scale in which song is lying
i know that in particular scale having 1221 (major minor funda)
i know the triad funda through that chords are formed.
i found that for particular song suppose
for G chord triad is G B D
but particular song notation i found i there are only"D" note is there then also he took G chord. how with single or two notes they r deciding chords.
please do my fude clear