How do you spell 'my' in Welsh?

The Welsh Word for my is fy, but it often modifies the Word following it. My father is

fy nhad. Father is tad. Only Words beginning with p, t, c, b, d. and g are affected.

Ll, rh, and m are not. My mother fy mam.

These 'mutations' are regular features in Celtic languages. Compare Irish mac (son)

and mo mhac (my son).