Complex sentence: Next she participated in the World championships in St. Moritz, Switzerland when she won silver medal in giant slalom and gold medal in slalom, her third in the row in World championships, which makes her the first woman to do this since 1939.
Split into simpler sentences: Next she participated in the World championships in St. Moritz, Switzerland where she won silver medal in giant slalom and gold medal in slalom. The gold medal in slalom was her third in the row in World championships, which makes her the first woman to do this since 1939.