"The Mediterranean sea is not cold."
Translation:Dyw Môr y Canoldir ddim yn oer.
Dydy and Dyw are both different forms of the negative verb "To be" for "he/she/it".
Dyw hi = Dydy hi
Dyw o = Dydy o
The only difference here is that instead of a pronoun a noun is used instead.
Because the Welsh "noun-noun" construct is automatically definite.
You can translate "A y B" into "the A of the B", adding "the" at the front and "of" between the two. (Or "the B's A", as appropriate, switching the two.)
Thus Môr y Canoldir is not merely "Sea the Middle-Lands" but "The Sea of the Middle-Lands" = "The Mediterranean Sea".