"I have never been overseas."
Translation:Dw i erioed wedi bod dramor.
Not quite though the translate to the same word in English. A simplified rule is that "Erioed" is used with the past and "Byth" is used with the other tenses. Both are followed by a mutation but they technically don't cause the mutation, also "byth" normally doesn't mutate. All information form "Y Cyfeiriadur".