what it is 是宾语从句，是宾语补足语，I did是定语从句，其中省略了一个that，是修饰that的，完整的应该是It is that I did
That 主语is 谓语what I want to tell you表语从句表语从句的直接宾语what 表语从句的主语I表语从句的谓语 want to tell 表语从句的间接宾语 you
修饰what it is
