I suggest that you review the lesson notes for the "Opinions 2" lesson: https://www.duolingo.com/skill/cy/Opinions2
- (fy) mod i for "that I am"
- (dy) fod ti for "that you are"
- (ei) fod o/e for "that he is"
- (ei) bod hi for "that she is"
- (ein) bod ni for "that we are"
- (eich) bod chi for "that you are"
- (eu) bod nhw for "that they are"
The first possessive word is often left out but the mutation it causes is often retained.
(Though some simplify this even more and say things such as bo' fi or bo' ti rather than mod i, fod di.)
You can access the notes on a smartphone by using the free Puffin browser.