Complex sentence: In July 1924, construction began on a new arena, on the site of a former roller-skating rink known as the Forum; by the fall, at a cost of $1.5 million, the Montreal Forum was complete.
Split into simpler sentences: In July 1924, construction began on a new arena, on the site of a former roller-skating rink known as the Forum. By fall, at a cost of $1.5 million, the Montreal Forum was complete.