It做形式主语,代替由不定式,动名词短语或是从句所表示的真正的主语.如It’s no use talking to him about it.It做形式宾语,代替一个由不定式,动名词,或是宾语从句,如I think it no use arguing with him.it可以指季节,时间,距离 如It was mid summer.It’s only five miles.同时在强调句中,It + be + 要强调的部分+ that(who,whom) + 句子其他部分 .这个句型很重要,记住即可.that有些固定的搭配 如 so that ,such that.that 用作连词,引导名词性从句,状语从句和强调句.1.that名词性从句.①引导宾语从句.及物动词后的引导词that可省略.I didn't expect (that) he could win the championship.The teacher pointed out that Tom was not working hard enough.② 引导主语从句.通常采用it作形式主语的句型.That the earth goes around the sun is known to everyone.（It is known to everyone that the earth goes around the sun.） ③引导表语从句.The trouble is that we are short of money.④引导同位语从句.引导同位语从句的that和引导定语从句的that是不同的.前者只起语法作用,在从句中不作任何成分；而后者在定语从句中作主语、宾语或表语.举例说明：The news that he resigned from office surprised us.The idea that he holds is very common nowadays on campus.2.that引导状语从句 ①引导目的状语从句.Bring it nearer that I may see it better.②引导结果状语从句.What have I done that he should be so angry with me?③引导原因状语从句.I am afraid that I will fail in the driving test.④引导让步状语从句.意为“虽然、尽管”.Difficult that/as the task was,they managed to accomplish it on time.⑤引导条件状语从句.意为“假使、假设”.Supposing that you were in my position,what would you do?On condition that you were lost in the desert,you should ask for help as soon as possible.
