【1】What cost him his life 【主语】//was 【系动词】//that he was too careless【表语】
备注(1)：what cost him his life 为融合型连接代词what引导的主语从句(what=the thing that)，并在主句中作主语成分。
备注(2)：was为系动词。
备注(3):that he was too careless 为从属连词that引导的表语从句，并在主句中作表语成分。
以上“主句”的结构为A was B=A是B【主系表】
A=What cost him his life【主语从句】 B=that he was too careless【表语从句】
备注：以上主句的“主语”和主句的“表语”都是由“从句充当”。
【2】What I want to know【主语】// is 【系动词】//where you found my book【表语】
备注(1):what I want to know为为融合型连接代词what引导的主语从句(what=the thing that)，并在主句中作主语成分。
备注(2):is为系动词。
备注(3):where you found my book为连接副词where引导的表语从句，并在主句中作表语成分。
以上“主句”的结构为A is B=A是B【主系表】
A=What I want to know【主语从句】 B=where you found my book【表语从句】
备注：以上主句的“主语”和主句的“表语”都是由“从句充当”。
