名词性从句中that的用法什么时候that可以省略

一．that 引导主语从句时,that 没有意义,但不能省略.（that 引导宾语从句时可以省略.）
For example:
It worried her a bit that her hair was turning gray.她的头发正在变白,这使她很不安.that引导主语从句,it 作形式主语,that 不能省略.
That she is a rich woman is known to us all.众所周知,她是个富有的女人.这是that 引导主语从句,that 不能省略.这句话可以改为用it 作形式主语的句型.请同学们改写：It is known to us all that she is a rich woman.）
We didn’t know (that) you had sold your house.我们不知道你已经把你的房子卖了.这是that引导宾语从句,that 可以省略.注意这句话的时态.
二.宾语从句.
1． that 引导的宾语从句:that 没有意义,在口语或非正式文体中常省略
例如：
I really feel she’s making a mistake.我的确感到她正犯错误.
James said (that) he was feeling better.詹姆斯说他感到好些了.
Hearing that his son was badly wounded,he hurried to the hospital to see him.听说儿子受了重伤,他急忙赶到医院去看望他.
I suggested that we should go home.我建议我们回家去.虚拟语气,that 一般不能省略.
2．whether/if (是否)引导的宾语从句：如果要突出“究竟是…还是不…”,常在whether 后面加or not; if 一般不与or not连用.如：
He asked me if/whether I knew John.他问我是否认识约翰.
Let me know whether you can come or not.你能来还是不能来,告诉我一声.
三.、 同位语从句
同位语从句是对名词的内容给予具体、详细的说明.常在后面接同位语从句的名词有fact,news,idea,truth,hope,suggestion,question,problem,doubt,fear,belief等.同位语从句常用的引导词为that,有是也用when,where 等疑问词.
如：
The news that the United States was hit by terrorist attacks took the whole world by surprise.美国受到*分子袭击的消息令全世界吃惊.
The idea that you can do this work well without thinking is quite wrong.你认为不动脑筋就能做好这件工作的想法是完全错误的.
People used to hold the belief that the earth was the center of the universe.人们曾认为地球是宇宙的中心.
The difficulty lies in the fact that we are short of money.困难在于我们缺乏资金这个事实.
They have no idea at all where he has gone.他们一点儿也不知道他去哪儿了.
注意：同位语从句的that 只是引导词,没有其他语法作用,在句子中不作句子成分,不能省略；而定语从句中的that 除了引导定语从句外,还是定语从句的一个成分,在定语从句中作主语或宾语,作主语时不能省略,作宾语时可以省略