我們先來梳理一下it引導(dǎo)的形式主語句型有哪些：

動(dòng)詞不定式，動(dòng)名詞或主語從句作主語時(shí)，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(dòng)詞(seem, appear, happen等) + 主語從句, 這類不及物動(dòng)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(dòng)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(shí)候?yàn)榱藦?qiáng)調(diào)不定式動(dòng)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(dòng)詞不定式，這類形容詞常是表示心理品質(zhì),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(gòu)成主系表結(jié)構(gòu)。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(dòng)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(dòng)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(shí)間(金錢)+ 動(dòng)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(dǎo)的強(qiáng)調(diào)句型結(jié)構(gòu)為：It is + 被強(qiáng)調(diào)部分 + that …

該句型是強(qiáng)調(diào)句型，將被強(qiáng)調(diào)的部分放在前面，其他部分置于that之后。被強(qiáng)調(diào)部分可以是主語，賓語，表語或狀語。如果把那種句型結(jié)構(gòu)劃掉后，應(yīng)該是一個(gè)完整無缺的句子。這也是判斷強(qiáng)調(diào)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(gè)it引導(dǎo)的特殊句型為 it is not until + 被強(qiáng)調(diào)部分 + that …

該句型主要用于強(qiáng)調(diào)時(shí)間狀語，譯成漢語“直到…才…”，可以理解為 not … until 的強(qiáng)調(diào)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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