"A good teacher"
Does the d double to dd+a for an adjective after a female noun? (Weirdly, I translated athrawes da and it was meant to be dda, but now the answer above was changed to athro da after I opened this box to ask the question.)
In Welsh we have a number of rules called mutations. One of these is called a soft mutation which affects nine letters (T,C,P,D,G,B,Rh,M,Ll). One instance where this occurs is adjectives after a feminine noun. So, since "Athro" is a masculine noun the "A good teacher(male)" is "Athro Da" but since "Athrawes" is a feminine noun "A good teacher(female)" is "Athrawes dda". I'll put a link to a mutations grid below.