Complex sentence: The Laurentian Library was commissioned in 1523 and construction began in 1525; however, when Michelangelo left Florence in 1534, only the walls of the reading room were complete.
Split into simpler sentences: The Laurentian Library was commissioned in 1523 and construction began in 1525. However, when Michelangelo left Florence in 1534, only the walls of the reading room were complete.