Simple musical structure

I have been gradually reversing the hiatus in my attempting to play music. With the treble clef part of Cwm Rhondda this evening I found that for the first Feed me till I want no more each word that I sing on the same E♭ has the concurrent lower note's pitch moving up and down from word to word. Not only is this richness unanticipated by me but it is interesting to include the changing lower notes and hear what effect they have on the overall tune.

(I confess that to my ignorant eyes uttering the Ydyw'r Un a'm cwyd i'r lan version of that line looks to be at risk of awakening Cthulhu.)