"Mae'r ddannoedd yn ofnadwy."

Translation: Toothache is terrible.

[ChrisMorga15]

Should this be "the toothache" as there is a "'r"?

[ibisc]

Not in this case. The yr/y/'r is not always translated between Welsh and English, particularly with names of things, including some diseases, some countries and some specific events. For example:

• yr Almaen, y Swistir, yr Aifft - Germany, Switzerland, Egypt
• y ddannoedd, y diciáu - toothache, tuberculosis
• y Pasg, y Nadolig - Easter, Christmas
• y Bala, y Rhyl, y Fenni, y Barri, y Bontfaen - Bala, Rhyl, Abergavenny, Barry, Cowbridge (names of towns)

Sometimes the y/yr is dropped in informal usage, though.

[rmcode]

As is well explained above. Therefore 'Toothache' (without the definite article in English) is 'Y Ddannoedd' (with the definite article in Welsh)

[ffwlbart]

Depending on context the English might use the definite article - for example 'the toothache is terrible but the earache is worse' or 'the toothache is terrible although the swelling has gone down'; but also: 'toothache is terrible and so is earache'.

[Froggy_]

I agree wholeheartedly, Ffwlbart.
"Toothache is awful" would be used when talking about the general condition, but when talking about a specific case, it would be "the toothache is awful", therefore both translations should be accepted.

[Cynphony]

This doesn't make sense to me without an article.
The toothache is terrible.
A toothache is terrible.
My toothache is terrible.

[eAnnAe]

But in a general sense, toothache is terrible... and no article is needed in English but it is required in the Welsh translation as above.