主語從句通常由下列詞引導：

1)從屬連詞that，whether,if等;

2)連接代詞what，who，which，whatever，whoever，whom 等;

3) 連接副詞how，when，where，why 等。

that在句中無詞義，隻起連接作用;連接代詞和連接副詞都在句中既保留自己的疑問含義、又起連接作用，在從句中充當主語的成分。注：whom、who指人，what指物，whatever，whoever表示泛指意義。例如：

What he wants to tell us is not clear. 他要跟我們說什麼，還不清楚。

Who will win the match is still unknown. 誰能赢得這場比賽還不得而知。

It is known to us how he became a writer. 我們都知道他是如何成為一名作家的。

Where the English evening will be held has not yet been announced. 英語晚會将在哪裡舉行，還沒有宣布。

有時為避免句子頭重腳輕，常用形式主語it代替主語從句作形式主語放于句首，而把主語從句置于句末。主語從句後的謂語動詞一般用單數形式。常用句型如下：

(1)It be 名詞 that從句

It's a great pity(that)they didn't get married.

他們沒能結婚，真是令人惋惜。

It 's a good thing(that)you were insured.

你保了險，這可是件好事。

　　(2)It be 形容詞 that從句

It's splendid that you passed your exam.

你通過考試了，真棒。

It's strange that there are no lights on.

真奇怪，沒有一盞燈是開着的。

(3)It be 動詞的過去分詞 主語從句

It's said that he has been there many times.

據說他去過那兒很多次。

　　(4)It 不及物動詞 that 從句[1]

另注意在主語從句中用來表示驚奇、不相信、惋惜、理應如此等語氣時，謂語動詞要用虛拟語氣“(should) do”，常用的句型有：

It is necessary (important, natural, strange, etc.) that …

It is suggested (requested, proposed, desired, etc.) that…

(5) it seems/happens that 從句

衆所周知的幾種表達方式

(1)It is known to us that.

(2)As is known to us.

(3)what is known to us is that.

it引導的強調句結構：It is/was 被強調部分 that/who 句子其它部分。

eg:My father did the experiment in the lab yesterday evening.

強調主語：It was my father who did the experiment in the lab yesterday evening.

強調賓語：It was the experiment my father did in the lab yesterday evening.

強調時間：It was yesterday evening that my father did the experiment in the lab.(注意不用when)

強調地點：It was in the lab that my father did the experiment yesterday evening.

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