There are two ways of saying could have in Welsh, using gallwn (south) and medrwn (north). It’s best to be aware of both, and let local usage dictate which one you use.
Use wedi , not yn . Verbnouns such as cwympo , to fall , and the negative particle dim do not take soft mutation after wedi . Gallwn is sometimes used with the affirmative particle fe which does cause soft mutation.
Affirmative
English Welsh Alternative Examples I could have gallwn i fod wedi fe allwn i fod wedi gallwn i fod wedi cwympo you could have gallet ti fod wedi fe allet ti fod wedi gallet ti fod wedi cwympo he could have gallai fe fod wedi fe allai fe fod wedi gallai fe fod wedi cwympo she could have gallai hi fod wedi fe allai hi fod wedi gallai hi fod wedi cwympo Carys could have gallai Carys fod wedi fe allai Carys fod wedi gallai Carys fod wedi cwympo the girls could have gallai’r merched fod wedi fe allai’r merched fod wedi gallai’r merched fod wedi cwympo we could have gallen ni fod wedi fe allen ni fod wedi gallen ni fod wedi cwympo you could have gallech chi fod wedi fe allech chi fod wedi gallech chi fod wedi cwympo they could have gallen nhw fod wedi fe allen nhw fod wedi gallen nhw fod wedi cwympo
Interrogative
English Welsh Alternative Examples could I have? allwn i fod wedi? – allwn i fod wedi cwympo? could you have? allet ti fod wedi? – allet ti fod wedi cwympo? could he have? allai fe fod wedi? – allai fe fod wedi cwympo? could she have? allai hi fod wedi? – allai hi fod wedi cwympo? could Carys have? allai Carys fod wedi? – allai Carys fod wedi cwympo? could the girls have? allai’r merched fod wedi? – allai’r merched fod wedi cwympo? could we have? allen ni fod wedi? – allen ni fod wedi cwympo? could you have? allech chi fod wedi? – allech chi fod wedi cwympo? could they have? allen nhw fod wedi? – allen nhw fod wedi cwympo?
Negative
English Welsh Alternative Examples I could not have allwn i ddim bod wedi – allwn i ddim bod wedi cwympo you could not have allet ti ddim bod wedi – allet ti ddim bod wedi cwympo he could not have allai feddim bod wedi – allai feddim bod wedi cwympo she could not have allai hi ddim bod wedi – allai hi ddim bod wedi cwympo Carys could not have allai Carys ddim bod wedi – allai Carys ddim bod wedi cwympo the girls could not have allai’r merched ddim bod wedi – allai’r merched ddim bod wedi cwympo we could not have allen ni ddim bod wedi – allen ni ddim bod wedi cwympo you could not have allech chi ddim bod wedi – allech chi ddim bod wedi cwympo they could not have allen nhw ddim bod wedi – allen nhw ddim bod wedi cwympo