dylwn i fod wedi

Building it. Start with a form of dylwn i (should) or gallwn i (could), then add fod wedi + verb-noun:

• Dylwn i fod wedi gofyn — I should have asked
• Dylet ti fod wedi dweud — you should have said
• Dylai hi fod wedi cyrraedd — she should have arrived
• Gallen ni fod wedi ennill — we could have won
• Gallech chi fod wedi cael damwain! — you could have had an accident!

Both verbs have the same endings: -wn i, -et ti, -ai fe (fo) or hi, -en ni, -ech chi, -en nhw.

Why fod? A verb-noun that comes straight after the person of a short verb form takes soft mutation, so bod becomes fod, just as in Dylwn i fynd (I should go).

Questions and negatives. Questions soften the first letter: Ddylwn i fod wedi gofyn? (Should I have asked?), Allet ti fod wedi helpu? (Could you have helped?). Negatives add ddim, and bod does not change after ddim: Ddylet ti ddim bod wedi dweud hynny (You shouldn't have said that).

Other forms you will hear. Speech often softens gallwn to allwn even in statements. The North also says medrwn i fod wedi for could have. Many speakers say dylswn i or dylsen i with the same meaning as dylwn i. "Would have" is baswn i wedi.

Must have is different: it is a confident guess, not a missed chance. Use rhaid bod: Rhaid bod e wedi anghofio (He must have forgotten).