The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
p. 70 p. 71
The three first words of the Cymraeg: the Name of God, that is O I U; the name of the Sun, perception, and sensation, that, is SULW; and Bo, others say, BYW.
The Name of God is a substantive verb; the sun is a substantive noun; and sulw is a substantive adjective--which was clear before the perfect Cymraeg was lost.