I think the rule is whenever a word that ends in m is made plural and s is added, the m changes to n.
I really don't understand this as "bom" means good.
I understand it now, bons is the plural of bom.
Oh yes, and boa/boas for feminine words ;)
i wrote "Os livros sao boas" why is that wrong.
Use boas feminine, plural nouns. Livros is a masculine, plural noun. As revistas são boas. (=the magazines are good).
So is Dia a masculine word?