Complex sentence: He went to Paris in 1666, where he worked with his fellow Fleming Nicolas Pitau the elder, and then to improve himself further studied under François de Poilly, Robert Nanteuil, and Philippe de Champaigne.
Split into simpler sentences: He went to Paris in 1666, where he worked with his fellow Fleming Nicolas Pitau the elder. To improve himself further he subsequently studied under François de Poilly, Robert Nanteuil, and Philippe de Champaigne.