Complex sentence: In July 1836, Van Buren was promoted to captain of the 1st Dragoon Regiment; he resigned his commission the day before his father's inauguration to become the president's private secretary in the White House.
Split into simpler sentences: In July 1836, Van Buren was promoted to captain of the 1st Dragoon Regiment. In 1837, he resigned his commission the day before his father's inauguration to become the president's private secretary in the White House.