從屬連詞中的that是引導(dǎo)名詞性從句的,如引導(dǎo)主語(yǔ)從句、表語(yǔ)從句、賓語(yǔ)從句、同位語(yǔ)從句.that在從句中不充當(dāng)句子成分,也沒(méi)有意義.引導(dǎo)賓從的that有時(shí)可以省略.引導(dǎo)其它三種從句的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引導(dǎo)主從,不可省略; it作形式主語(yǔ)）
That he was able to come made us happy.(that引導(dǎo)主從,不可省略)

He told me that he would come. （that引導(dǎo)賓從,可以省略)
He told me that he would come and that he would come on time.（that引導(dǎo)賓從,第一個(gè)that可以省略,但第二個(gè)that不可以省略.)

The truth is that I didn’t go there. (that引導(dǎo)表從,不可省略)
The news that our team has won is true. (that引導(dǎo)同從,不可省略)


關(guān)系代詞中的that是引導(dǎo)定語(yǔ)從句的,指物或人,在句中要充當(dāng)句子成分：可作主語(yǔ)、賓語(yǔ),作賓語(yǔ)時(shí)可以省略.如：
The bag that lies on the ground is hers.（that作主語(yǔ),不能省略）
This is the book (that) I'm looking for.（that作賓語(yǔ),可以省略）