"I am the Head of Year Eight."
Translation:Fi yw Pennaeth Blwyddyn Wyth.
In such a construction (where you have two nouns together), the first noun is implicitly definite.
You can often translate A B (e.g. Pennaeth + Blwyddyn Wyth) as "the A of B" (here: the Head of Year Eight).
Sometimes, "B's A" makes more sense, e.g. pêl Alis as "Alis's ball" rather than "the ball of Alis".
So in a way, the Welsh doesn't have a word for "the" in that construction for the same reason that English doesn't have one in "Alis's ball" -- we say neither "the Alis's ball" nor "Alis's the ball" -- the construction with possessive already marks the ball as definite and so "the" is neither necessary nor possible. And similarly in Welsh, only that the nouns are in the other order (ball Alis rather than Alis('s) ball).