"Do you doubt me?"
Translation:Dych chi'n fy amau i?
When a pronoun is a direct object of a verb the possessive pronoun is used.
Fy.............i = mine
Literally this is Are you my doubting
This is often contracted by leaving off the second part of the possessive pronoun, although the second part will be used for emphasis
Dych chi'n fy amau? = Are you doubting me?
Dych chi'n fy amau I?! = Are you doubting ME?!
A variant in speech is:-
Dych chi'n amau fi?
This is not strictly grammatically correct but is easier to use, especially for learners.