Complex sentence: Since the NFL was founded in 1920 by George Halas and Bert Bell, it has grown from an informal network of teams based mostly in small towns and cities into the most popular and successful sports league in the United States.
Split into simpler sentences: Since the NFL was founded in 1920 by George Halas and Bert Bell. It has grown from an informal network of teams based mostly in small towns and cities into the most popular and successful sports league in the United States.