Complex sentence: He worked as a stonemason with his father in 1815 and assisted with the publication of his father's work "Cyfrinach Beirdd Ynys Prydain" ("Mystery of the bards of the island of Britain") in 1829.
Split into simpler sentences: He worked as a stonemason with his father in 1815. He assisted with the publication of his father's work "Cyfrinach Beirdd Ynys Prydain" ("Mystery of the bards of the island of Britain") in 1829.