Complex sentence: She opened a public stable in 1929, where she began to teach riding lessons as a professional, and when she became concerned that she might not be able to afford to keep her horses during the Great Depression, she also began to write books about horses.
Split into simpler sentences: She opened a public stable in 1929, where she began to teach riding lessons as a professional. When she became concerned that she might not be able to afford to keep her horses during the Great Depression, she also began to write books about horses.