"I have seen a whale in the Atlantic."

Translation: Dw i wedi gweld morfil ym môr Iwerydd.

[Catherine-Brown]

Oh dear - why wouldn't it be mewn y mor iwerydd>
?

[rmcode]

Mewn is used when you want to say in 'a', so 'mewn mor iwerydd' would be 'in an atlantic ocean', however you should be able to use 'yn' there; yn y mor iwerydd, I'll check to see if it's been added.