Complex sentence: The largest paper mill in the world, which Mechanicville had hosted since 1904, ceased operations in 1971, and the once thriving industrial city is today a quiet residential city, with most inhabitants working in Albany, Schenectady, and other nearby communities.
Split into simpler sentences: The largest paper mill in the world which Mechanicville had once hosted (in 1904), ceased operations in 1971. The once thriving industrial city is today a quiet residential city, with most inhabitants working in Albany, Schenectady, and other nearby communities.