Complex sentence: Union Course was a United States horse racing course in what is now Woodhaven, Queens, New York City, which hosted some of the most famous horse races in American history, including the 1823 match between American Eclipse and Sir Henry.
Split into simpler sentences: Union Course was a horse racing course in what is now Woodhaven, Queens, in New York City. It hosted some of the most famous horse races in American history, including the 1823 match between American Eclipse and Sir Henry.