"He's a boy."
Translation:Hogyn ydy o.
Mae: positive statement, third person ('maen' when coupled with 'nhw').
Ydy: third person question, third person emphatic statement (especially in the north).
Yw: third person emphatic statement only (mainly south).
"Bachgen yw e" was accepted as another correct answer, and this is south dialect I think
I thought Ydy was used if asking a question. I'm still not sure, even after reading these responses, why "Hogyn mae o." is incorrect. I thought I had it 'down,' when to use Mae, Dydy, Ydy. Now I'm confused again... :-(
"Ydy" (or "yw") is also used in the "emphatic" form of the sentence, as in this example.
That is because dyw is a negative form. This is explained in the course notes for earlier sections of the course.
"Bachgen yw e" and "Hogyn ydy o" are both accepted. Can "e" and "o" be used interchangeably or is that also dependent on the dialect or something else?
It's a dialect thing. 'E' tends to be used in the areas that use 'bachgen' and 'o' where they use 'hogyn'.
bachgen is used in quite a few parts of north-east and mid-Wales where o/fo is the usual masculine pronoun. It originally a north-east Wales word, apparently.
The rough boundaries between the various dialects are not very consistent when it comes to particular words and phrases.
It doesn't have to be question form only. In emphatic patterns "ydy" can be used alongside "yw" with the same usage and meaning. ("yw" can never be used when forming questions without a questioning word though - you have to use "ydy" there, so I guess that's what you were thinking of.)