名詞性從句的功能相當於名詞詞組。按其句法功能，名詞性從句可分為主語從句、表語從句、賓語從句和同位語從句。引導名詞性從句的有：連接詞that, whether和if，關係代詞who, whom, whose, what, which, wahtever, whichever, whoever以及關係副詞when, where和how。
主語從句1. 主語從句在句中作主語。如：
Which team will win the match is still not certain.
Why he did that wasn’t quite clear.
2. 主語從句位於句首時，會使句子顯得笨重，因此常以it作為形式主語，而將主語從句後置，以保持句子平衡。如：
It is strange that she didn’t come yesterday.
3. whether與if引導主語從句的區別：
whether引導的主語從句既可放在句首也可放在句尾，if引導的主語從句只能放在句末。如：
Whether she comes or not makes no difference.
It is doubtful whether / if the President knew the details of the plan.
表語從句1. 表語從句在句中作表語，位於主句中的系動詞之後。如：
The question is whether she can speak English.
That is what we need.
2. 表語從句可用whether引導，也可用as if引導，但不能用if引導。如：
His first question was whether Tom had arrived yet.
It looked as if it was going to rain.
3. 主句的主語是reason時，表語從句要用that，不能用because。如：
The reason why he was dismissed was that he was careless and irresponsible.
同位語從句1. 同位語從句在句中作同位語，它一般要放在主句中某些名詞的後面，說明這些名詞的含義。可跟同位語從句的名詞主要有fact, news, promise, reason, idea等。如：
We heard the news that our team had won.
I have no idea how soon they are coming.
2. 同位語從句與定語從句的主要區別：
從意義上來說，同位語從句對一個名詞加以補充說明，定語從句對一個名詞進行修飾或限定。從結構上說，同位語從句由連詞引導，連詞在從句中不作任何成分，只起引導作用;引導定語從句的關係代詞在從句中擔任一定成分，含有代替所修飾或限制詞語的意義。如：
They were all shocked at the news that Germany had declared war on Russia. 他們都為德國向俄國宣戰而感到震驚。(同位語從句，that只起引導作用，在句中不作任何成分。)
They were all shocked at the news that was announced on the radio. 他們都為收音機裡宣布的消息而震驚。(定語從句，that在句中作主語，若省略that，句子成分不全。)
賓語從句1. 賓語從句在句中作及物動詞或介詞的賓語。如：
I wonder who he is.
He is sorry for what he has done.
注意：在口語及非正式文體中，that常省略。
2. 使用賓語從句時應注意時態呼應。
1)主句謂語動詞是現在時或將來時，從句謂語動詞可以用任何所需要的時態。
2)主句謂語動詞是過去時態，從句謂語動詞一般須用過去時。如：
I thought he studied hard.
3) 從句如果與某一具體的過去時間狀語連用，儘管其謂語動作發生在主句謂語動作之前，仍用一般過去時。如：
She told me that her father died in 1991.
4) 從句所說明的是一般真理或客觀事實，即使主句用過去時，從句仍用一般現在時。
The teacher told the students that the computer is a wonderful machine.
3. 不能省略that的賓語從句
1)句中的動詞後接兩個或兩個以上的that從句作賓語時，第一個that可省，但後面的that不可省。如：
He said (that) he couldn’t tell you right away and that you wouldn’t understand.
2) 當主句的謂語動詞與that賓語從句之間有插入語時，that一般不可省。如：
Just then I noticed, for the first time, that our master was wearing his fine green coat and his black silk cap.
3) 當that從句是雙賓語中的直接賓語時，that不可省。如：
I must never tell anyone that I could not see the cloth.
4) 當that賓語從句前有it作形式賓語時，that不可省。如：
We think it important that we study hard.
4. whether和if引導的賓語從句可以互換使用，但介詞後的賓語從句只能用whether，不能用if。如：
He asked me if / whether my brother was at home.
It is a question of whether we should go.
♢ 2013年12月英語六級語法訓練匯總
（實習編輯：張心悅）