"Yes, I will be in America next month."
Translation:Bydda, bydda i yn America mis nesa.
'...next month' is an adverb describing when you will be doing something. In 'good' Welsh, adverbs of time are usually mutated, so here fis nesa is the mutated form of mis nesa. This type of mutation is sometimes omitted in casual usage.
I thought Fydda only used for question or negative therefore reply is Bydda for "Yes, I will be" or Na Fydda for "No, I will not be". Can you confirm