In US English, the term, 'your good clothes' means more formal clothes. Does 'good shirt' have the same meaning?
This is more of an informal expression. It's the equivalent of saying to someone 'nice shirt' as a compliment.
It could also mean that a particular shirt is of good quality.
"Da" is the base form of the word. In Welsh the first letters of some words can mutate i.e change the first letter. The most common type of this is a soft mutation and in the case of "D" it becomes "Dd". Once cause of soft mutation is after a feminine noun. "Crys" is a masculine noun so it's "Crys da". "Sgert" is a feminine noun so it's "Sgert dda".
Thanks for the explanation @EllisVaughan. I haven't learnt what "Sgert" means in English yet. Plaese can you tell me?
I just Google translated it and it came up with "Skirt". In case anyone else who hadn't got to that yet, was wondering too.