Complex sentence: Lillström was not only an actor, but also had a position in the management of the theatre; after the theatre became a private company in 1740, it was ruled by a board of directors made of the most important members of the staff.
Split into simpler sentences: Lillström was not only an actor, but also had a position in the management of the theatre. After the theatre became a private company in 1740, it was ruled by a board of directors made of the most important members of the staff.