Complex sentence: During the 1920s, he became involved in the real estate market, in which he made a fortune, but he lost most of his money when the market took a sudden downturn in the Great Depression.
Split into simpler sentences: During the 1920s, he became involved in the real estate market, where he made a fortune. Unfortunately, he lost most of his money when the market took a sudden downturn due to the Great Depression.