Because it is a negative form of the verb - that is indicated by the mixed mutation (aspirate mutation of p, t, c, soft mutation of b, d, g, m, ll, rh)
Thanks - I haven't got my head round this aspect fully yet but I guess it's along the lines of 'byddwch' (positive) and 'fyddwch' (negative) so that makes sense
You could choose a suitable list of verb-nouns and then write out a list of their various verb forms to pin on the wall somewhere - prynu, canu, talu, barnu, deall, galw, ...
- prynais i, brynais i?, phrynais i ddim
- canais i, ...
Mae gwall yn y Saesneg, wedi wneud adroddiad.
There's a mistake in the English, have reported.