Translation:Do, es i i'r banc.
Why is it 'do' rather than 'ydw' in this example?
This is explained in the notes for the section 'Past Tense 2'.
In the simple past tense, do is used for 'yes' and naddo for 'no', regardless of whether it is I, you, they, etc.
There is a fuller description of yes/no in Welsh here