"Cyfandir mawr ydy De America."

Translation: South America is a big continent.

[flinnian]

Why is it structured like this? i.e. not Mae De America ...... etc. ?

[StephenDunscombe]

If your sentence is "X is Y", where X and Y are both nouns, AND if "X" is definite or a proper noun, then the sentence structure kicks over to "Y ydy X" or "Y yw X".

It's why we're taught "Steve dw i" as the proper way to say "I am Steve." Rather than "Dw i'n Steve."

Have a look at a couple of Welsh Wikipedia articles. They all tend to start out "(Long explanation of what the thing is) yw ARTICLE-TITLE."