Complex sentence: The Hart-Celler Act abolished the national origins quota system that was American immigration policy since the 1920s, replacing it with a preference system that focused on immigrants' skills and family relationships with citizens or U.S. residents.
Split into simpler sentences: The Hart-Celler Act abolished the quota system based on national origins that had been American immigration policy since the 1920s. The new law replaced the quota system with a preference system that focused on immigrants' skills and family relationships with citizens or U.S. residents.