Complex sentence: The "Keystone Cops" was a series of silent film comedies featuring a totally incompetent group of policemen produced by Mack Sennett for his Keystone Film Company between 1912 and 1917.
Split into simpler sentences: The "Keystone Cops" was a series of silent film comedies about a totally incompetent group of policemen. The movies were produced by Mack Sennett, for his Keystone Film Company, between 1912 and 1917.