Complex sentence: The most studied form of psychotherapy for depression is "cognitive behavioral therapy" (CBT), thought to work by teaching clients to learn a set of cognitive and behavioral skills, which they can employ on their own.
Split into simpler sentences: The most studied form of psychotherapy for depression is "Cognitive behavioral therapy" (CBT). CBT is thought to work by teaching clients to learn a set of cognitive and behavioral skills, which they can employ on their own.