Complex sentence: David Nash Ford suggests that he may have ruled jointly with his brother, Gwyddgi and that, when Cynfyn died, the Ergyng throne was usurped by Gwrfoddw, but eventually his son, Gwrgan Fawr, was restored.
Split into simpler sentences: David Nash Ford suggests that he may have ruled jointly with his brother, Gwyddgi. When Cynfyn died, the Ergynng throne was usurped by Gwrfoddw, but eventually his son, Gwrgan Fawr, was restored.