"I am fifteen years old."

Translation: Dw i'n un deg pump oed.

[silverlight513]

When does deg get mutated to ddeg in numbers such as un ar ddeg?

[EllisVaughan]

"Ar" is one of twelve major prepositions that cause soft mutation. These are: am, ar, at, dan, dros, drwy, heb, i ,o gan, wrth, hyd. (Don't worry too much about how each is used yet).

[AnCatDubh]

Why is the first translation given ‘mod i’?

As a matter of fact, what the heck is up with all those different copulæ?!