Since 2001, more than 1,000 high school and college students in the U.S. have participated in the Body Project, which works by getting girls to understand how they have been buying into the notion that you have to be thin to be happy or successful.
【分析】本句为复合句，主架为 students have participated in the Body Project。Since 2001为时间状语。which successful为非限制性定语从句，修饰的是the Body Project。by understand为方式状语。how they notion为宾语从句，作understand的宾语。that you successful为同位语从句，解释的是notion。buy into指接受并受(某种思想、观点等)的影响。