SimoneThat可以引导名词性从句，状语从句和强调句。
It was exactly what I needed是表语从句，是"主句+表语从句"构成的主从复合句,其旦举和中表语是由"what"引导的是表语从句来充当.这里"it" 作代词,当然"it"也可替代后面的答闷名词从句放在句首作形式主语,但这里却不成。
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。前者只起语法模盯作用，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.
扩展资料：
与that 有关的常见重要短语。
1、in that，意为“既然、因为”。
Criticism and self-criticism is necessary in that it helps us correct our mistakes.
2、now that，意为“既然、由于”。
Now that they have taken matters into their hands, the pace of events has quickened.
3、see (to it) that，意为“注意、务必做到、保证”。
We will see to it that she gets home early.
See to it that you are not late again.
4、seeing that，意为“鉴于、由于”。
Seeing that it is 8 o'clock, we'll wait no longer.
Seeing that he was busy with his work, I didn't disturb him.
参考资料：that-百度百科
