Complex sentence: He was urging his friends and proteges to create a new, more realistic art that would speak more directly to their time and experience and to seek out fresh, less genteel subjects in the modern American city.
Split into simpler sentences: He was urging his friends and proteges to create a new, more realistic art that would speak directly to their time and experience. He believed that it was the right moment for American painters to seek out fresh, less genteel subjects in the modern American city.