从属连词中的that是引导名词性从句的,如引导主语从句、表语从句、宾语从句、同位语从句.that在从句中不充当句子成分,也没有意义.引导宾从的that有时可以省略.引导其它三种从句的that一般不能省略.如：
It's known to us all that 2014 FIFA World Cup is taking place in Brazil.(that引导主从,不可省略; it作形式主语）
That he was able to come made us happy.(that引导主从,不可省略)

He told me that he would come. （that引导宾从,可以省略)
He told me that he would come and that he would come on time.（that引导宾从,第一个that可以省略,但第二个that不可以省略.)

The truth is that I didn’t go there. (that引导表从,不可省略)
The news that our team has won is true. (that引导同从,不可省略)


关系代词中的that是引导定语从句的,指物或人,在句中要充当句子成分：可作主语、宾语,作宾语时可以省略.如：
The bag that lies on the ground is hers.（that作主语,不能省略）
This is the book (that) I'm looking for.（that作宾语,可以省略）