The possessive pronouns are:
fy ... i
dy ... di
ei ... e/o
ei ... hi
ein ... ni
eich ... chi
eu ... nhw
NB The first part of the pronoun must match the second part; ei is the singular form and eu is the plural form, e.g.
Gwelais i eu llyfrau nhw. ✓
Gwelais i eu llyfrau hi. X