Complex sentence: After Martha Jefferson, his wife of eleven years, died in 1782, Jefferson remained a widower for the rest of his life after promising her he would never remarry; their marriage produced six children, of whom two survived to adulthood.
Split into simpler sentences: After Martha Jefferson, his wife of eleven years, died in 1782, Jefferson kept his promise to her that he would never remarry. Their marriage had produced six children, of whom two survived to adulthood.