"The Mediterranean sea is not cold."

Translation: Dyw Môr y Canoldir ddim yn oer.

[AnUnicorn]

How are dydy and dyw both valid here?

[EllisVaughan]

Dydy and Dyw are both different forms of the negative verb "To be" for "he/she/it".
I.E:
Dyw hi = Dydy hi
Dyw o = Dydy o
The only difference here is that instead of a pronoun a noun is used instead.

[RowenaJane]

Why is there no 'y' here for 'the'?

[mizinamo]

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".

[RowenaJane]

I am not sure I understand but thank you for answering.

[RowenaJane]

Mor y Goggled does not consist of two nouns so I am puzzled

[EllisVaughan]

Yes it does, "Môr" is a noun as is "Gogledd".

[mizinamo]

Literally "the Sea of the North", with "the north" a noun meaning "the area that is to the north of somewhere" (as in "he lives in the north").