Words beginning with t, c and p take an aspirate mutation after the pronoun ei (her),
e.g. ei chymhwyster, ei phroblem, ei thasg.

In this example, the ei is masculine and refers to 'Owain'.  Words take a soft mutation after the pronoun ei (his),
e.e. ei gyfrifoldeb, ei ddawn, ei lwy.

The table of mutations can be found by clicking here.