As with gwahanu in the same sentence, the verb-noun cyfaddef must take a soft mutation after the preposition i even though it doesn’t directly follow the i.  This is also the case when the preposition conjugates, e.g.

Aeth Efan i’r gwely ar ôl i ni gyrraedd adref.
Cofiais ar ôl iddo ddweud y stori eto.

The suffixes (endings) for the prepositions can be found by clicking here.

The table of mutations can be found by clicking here.

Cyfaddef i

In Welsh, many verbs are followed by specific prepositions. A comprehensive list of verbs followed by their correct prepositions may be found by clicking here.

Cyfaddef is always followed by wrth, which means 'to'. e.g.

Mae Huw wedi cyfaddef wrth ei dad mai o sydd wedi bwyta'r deisen.
Dydy Sara ddim yn mynd i gyfaddef wrth ei brawd ei bod hi’n hoffi rygbi.