我們先來梳理一下it引導的形式主語句型有哪些：

動詞不定式，動名詞或主語從句作主語時，通常用it作形式主語，放到句首，而真正的主語放在句尾，常見句型有如下：

1. It + be + 形容詞+主語從句，如：

It is uncertain whether he can come to Jenny’s birthday party or not.

2. It + be + 名詞詞組 + 主語從句，這類名詞有：a fact, a pity, a shame, an honor，a question等，如：

It’s a pity that you missed the exciting football match.

It remains a question whether he will come or not.

3. It + be + 過去分詞 + 主語從句，這類過去分詞有：announced, believed, expected, hoped, decided, reported, said, shown等， 如：

It is reported that 16 people were killed in the earthquake.

It is not decided who will give the operation to the patient.

4. It + 不及物動詞(seem, appear, happen等) + 主語從句, 這類不及物動詞有：appear, happen, seem等， 如：

It seemed that he didn’t tell the truth.

It happened that I was out when he called.

5. It + be + 形容詞 + for sb.)+ 動詞不定式

這類句型常用形容詞easy, difficult, hard, important, possible，wise等作表語，有時候為了強調不定式動作的執(zhí)行者，常在不定式前加for sb，如：

It’s necessary for the young to master two foreign language.

It is unwise to give the children whatever they want.

這類形容詞有：easy, difficult, hard, important, possible，wise

6. It + be + 形容詞 + of sb. + 動詞不定式，這類形容詞常是表示心理品質,性格特征的形容詞，如：kind, nice stupid, clever, foolish, polite, impolite, silly, selfish, considerate等。某人和這些形容詞可以構成主系表結構。如：

It’s very kind of you to help me with the work.

=You are kind to help me.

It seemed selfish of him not to share his dictionary with others.

=He seemed selfish not to share his dictionary with others.

7. It + be +名詞詞組 + 動詞不定式，如：

It is not a good habit to stay up too late.

8. It + be + 名詞或形容詞 + 動名詞，這類名詞和形容詞常常是：good, no good, no use, a waste of, useless, senseless等，如：

It’s a waste of time talking to her any more.

It is no use arguing about the matter with him.

9. It + take ( sb. )+ 時間(金錢)+ 動詞不定式，如：

It took the workers almost three years to finish building the dam.

It will take a whole day to get to the top of the mountain on foot.

而 it 引導的強調句型結構為：It is + 被強調部分 + that …

該句型是強調句型，將被強調的部分放在前面，其他部分置于that之后。被強調部分可以是主語，賓語，表語或狀語。如果把那種句型結構劃掉后，應該是一個完整無缺的句子。這也是判斷強調句型與其他從句的方法。

It was about 600 years ago that the first clock with a face and an hour hand was made.

It was they that cleaned the classroom yesterday.

另外一個it引導的特殊句型為 it is not until + 被強調部分 + that …

該句型主要用于強調時間狀語，譯成漢語“直到…才…”，可以理解為 not … until 的強調形式：

It was not until she took off her glasses that I realized that she was a famous film star.

= Not until she took off her dark glasses did I realize she was a famous film star.

= I didn’t realize she was a famous film star until she took off her glasses.

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