That可以引導名詞性從句，狀語從句和強調句。
It was exactly what I needed是表語從句，是"主句+表語從句"構成的主從復合句,其中表語是由"what"引導的是表語從句來充當.這裏"it" 作代詞,當然"it"也可替代後面的名詞從句放在句首作形式主語,但這裏卻不成。
that名詞性從句。
①引導賓語從句。及物動詞後的引導詞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 引導主語從句。通常采用it作形式主語的句型。
That the earth goes around the sun is known to everyone.
（It is known to everyone that the earth goes around the sun.）
③引導表語從句。
The trouble is that we are short of money.
④引導同位語從句。
引導同位語從句的that和引導定語從句的that是不同的。前者只起語法作用，在從句中不作任何成分；而後者在定語從句中作主語、賓語或表語。舉例說明：
The news that he resigned from office surprised us.
The idea that he holds is very common nowadays on campus.
擴展資料：
與that 有關的常見重要短語。
1、in that，意為“既然、因為”。
Criticism and self-criticism is necessary in that it helps us correct our mistakes.
2、now that，意為“既然、由於”。
Now that they have taken matters into their hands, the pace of events has quickened.
3、see (to it) that，意為“註意、務必做到、保證”。
We will see to it that she gets home early.
See to it that you are not late again.
4、seeing that，意為“鑒於、由於”。
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.
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