that 在普通的定语从句中可以省略如：I think （that ）you are right
但是如果that后面跟着两个从句,第一个that可以省略第二个不可以省略如：I said (that) I did it,and that I really love her
that 除了在定语从句中做成分,在其他从句中只起到引导作用,不做成分
that可以用在名词性从句中,比如主语从句,宾语从句,同位语从句,表语从句