Complex sentence: In 1919 he went to England to attend Dover College and then to study under the famous Léonide Massine and established a working relationship with the ballet troupe belonging to Marie Rambert and Ninette de Valois.
Split into simpler sentences: In 1919 he went to England to attend Dover College. He also established a working relationship with the ballet troupe belonging to Marie Rambert and Ninette de Valois.