the biggest mistake made by beginners when they just first stsrted exercising is not the diet,not the postures in liftin

the biggest mistake made by beginners when they just first stsrted exercising is not the diet,not the postures in lifting the weight,but their determination.4643 这句话中when they just first stsrted exercising is not the diet,怎么出现了两个谓语started 和is,帮讲一下这句话的语法结构