名詞性從句是由if, whether, that 和各種疑問詞充當連線詞所引導的從句，其功同名詞一樣。
一、主語從句
主語從句是在複合句中充當主語的從句，通常放在主句謂語動詞之前或由形式主語it代替，而本身放在句子末尾。
1. It 作形式主語和it引導強調句的比較
It 作形式主語代替主語從句，主要是為了平衡句子結構，主語從句的連線詞沒有變化。而it引導的強調句則是對句子某一部分進行強調，無論強調的是什麼成分，都可用連詞that。被強調部分指人是也可用who/whom。例如：
It is a pity that you didn't go to see the film.
It doesn't interest me whether you succeed or not.
It is in the morning that the murder took place.
It is John that broke the window.
2. 用it 作形式主語的結構
(1) It is +名詞+從句
It is a fact that …事實是…
It is an honor that…非常榮幸
It is common knowledge that…是常識
(2) it is +形容詞+從句
It is natural that…很自然…
It is strange that…奇怪的是…
(3) it is +不及物動詞+從句
It seems that…似乎…
It happened that…碰巧…
(4) it +過去分詞+從句
It is reported that…據報道…
It has been proved that…已證實…
3. 主語從句不可位於句首的五種情況
(1) if 引導的主語從句不可居於複合句句首。
(2) It is said , (reported) …結構中的主語從句不可提前。例如：
It is said that President Jingo will visit our school next week. (right)
That President Jiang will visit our school next week is said. (wrong)
(3) It happens…, It occurs… 結構中的主語從句不可提前。例如：
It occurred to him that he failed in the examination. (right)
That he failed in the examination occurred to him. (wrong)
(4) It doesn't matter how/whether …結構中的主語從句不可提前。例如：
It doesn't matter whether he is wrong or not. (right)
Whether he is wrong or not doesn't matter. (wrong)
(5) 含主語從句的複合句是疑問句時，主語從句不可提前。例如：
Is it likely that it will rain in the evening? (right)
Is that will rain in the evening likely? (wrong)
4. What 與that 在引導主語從句時的區別
What 引導主語從句時在句時在從句中充當句子成分，如主語.賓語.表語，而that 則不然。例如：
1) What you said yesterday is right.
2) That she is still alive is a consolation.
二、賓語從句
賓語從句就是在複合句中作賓語的名詞性從句，通常放在主句謂語動詞(及物動詞)或介詞之後。
1. 作動詞的賓語
(1) 由that引導的.賓語從句(that 通常可以省略)，例如：
I heard that be joined the army.
(2) 由what, whether (if) 引導的賓語從句，例如：
1) She did not know what had happened.
2) I wonder whether you can change this note for me.
(3) 動詞+間接賓語+賓語從句。例如：
She told me that she would accept my invitation.
2. 作介詞的賓語
例如：Our success depends upon how well we can cooperate with one another.
3. 作形容詞的賓語
例如：I am afraid (that) I've made a mistake.
That 引導的從句常跟在下列形容詞後作賓語：
Anxious, aware, certain, confident, convinced, determined, glad, proud, surprised, worried, sorry, thankful, ashamed, disappointed, annoyed, pleased, hurt, satisfied, content 等。也可以將此類詞後的that 從句的看作原因狀語從句。
4. It 可以作為形式賓語
It 不僅可以作為形式主語，還可以作為形式賓語而真正的賓語that 從句則放在句尾，特別是在帶複合賓語的句子中。 例如：
We heard it that she would get married next month..
5. 後邊不能直接跟that 從句的動詞
這類動詞有Allow, refuse, let, like, cause, force, admire, condemn, celebrate, dislike, love, help, take, forgive等。這類詞後可以用不定式或動名詞作賓語，但不可以用that引導的賓語從句。例如：
I admire their winning the match. (right)
I admire that they won the match. (wrong)
6. 不可用that從句作直接賓語的動詞
有些動詞不可用於“動詞+間接賓語+that從句“結構中，常見的有Envy, order, accuse, refuse, impress, forgive, blame, denounce, advise, congratulate等。例如：
He impressed the manager as an honest man. (right)
He impressed the manager that he was an honest man. (wrong)
7. 否定的轉移
若主句謂語動詞為Think, consider, wuppose, believe, expect, fancy, guess, imagine等，其後的賓語從句若含有否定意義，一般要把否定詞轉移到主句謂語上，從句謂語用肯定式。例如：
I don't think this dress fits you well.(我認為這件衣服不適合你穿。)
三、表語從句
表語從句在複合句中作表語的名詞性從句，表語是用來說明主語的身份、性質、品性、特徵和狀態的，表語常由名詞、形容詞、副詞、介詞短語、不定式、動詞的-ing、從句來充當，它常位於系動詞(be, become, appear, seem,look,sound,feel,get,smell等詞)之後。
引導表語從句的詞有連詞that, whether，連線代詞和連線副詞，關係代詞型what，以及as if, as though, because等連詞。
1. 由that引導
The fact is that he doesn’t really try. 事實是他沒有做真正的努力。
The trouble is that I have lost his address. 麻煩的是我把他的地址丟了。
My suggestion is that we should tell him. 我的建議是我們應該告訴他。
His sole requirement was (is) that the system work. 他唯一的要求是這個制度能起作用。
