在that引导的宾语从句中,连词that只起引导功能,无具体意义,不担当任何成分,因此在口语或非正式的文体中常可省略.然而,在下列情况下却不能省略.
一、从句的主语是that时,that不宜省略.如：
He says that that is a real king's hat．他说那是一个真的王冠.
二、若出现两个或两个以上的that宾语从句,且由并列连词连接时,只有第一个连词that可以省去,其余须保留.如：
She said（that）she would come and that she would also bring her son．她说她要来,还要带她的儿子来.
三、若that从句中含有主从复合句时,则that不宜省略.如：
I'm afraid that if you've lost it,you must pay for it．恐怕你丢失了它,就得赔偿.
四、主从句之间有插入语时,that不宜省略.如：