"The thief was caught."
Translation:Gaeth y lleidr ei ddal.
If the sentence was 'Bonnie Parker was caught' then the possessive would be feminine.
Gaeth Bonnie Parker ei dal
But if the sentence was 'The thief, Bonnie Parker, was caught', the possessive would be masculine to agree with the gender of the noun 'thief', which is the subject of the verb.
Gaeth y lleidr, Bonnie Parker, ei ddal