Complex sentence: The architecture of the Exposition was often a mixture of Art Nouveau and Beaux-Arts architecture: the main exhibit hall, the Grand Palais had a Beaux-Arts façade completely unrelated to the spectacular Art Nouveau stairway and exhibit hall in the interior.
Split into simpler sentences: The architecture of the Exposition was often a mixture of Art Nouveau and Beaux-Arts architecture. The main exhibit hall, the Grand Palais had a Beaux-Arts façade completely unrelated to the spectacular Art Nouveau stairway and exhibit hall in the interior.