18. It is necessary (for sb) to do sth

该句型与上一句统属一个句型。如果不定式的逻辑主语是由for 引出的，主句的形容词通常是表示重要性，紧迫性，频繁程度，难易，安全等情况的形容词。常见的形容词有：

Important,necessary,natural,easy,safe,common,,normal,hard,difficult,dangerous,unusual,grape, impossible, pleasant.等。

It is important for her to come to the party. =It is important that she (should) come to the party.

19. It looks (seems) as if …

该句型中it 无意义。As if 引导一个状语从句，常译为：“看起来好像……”如果与事实不相符合，则用虚拟语气。

It looks as if he is ill. (真的病了)

It looks as if he was ill. (事实上没有生病)

It seemed as if he was dying.

20. We think it important to learn a foreign language.

该句型中的it作形式宾语。为了记忆方便我们可称该句型为“7123结构”

7指主句中的常用的动词：think, believe, make, find, consider, feel, take.

1指的是形式宾语it .

2指的是宾补的两种形式：形容词和名词

3指的是真正宾语的三种形式：不定式短语，动名词短语或that引导的宾语从句。

We think it our duty to clean our classroom every day.

XiaoLi felt it important learning English well.

They found it difficult that they would finish their work in two days.

-Do you like ____here?

-Oh, yes .The air, the weather, the way of life. Everything is so nice. (2004 全国卷二)

A. this B. these C. that D. it

通常在系表结构前使用形式主语表示说话人对某情况的看法。如：

It was a great surprise to me when he did a thing like that. 他这样做使我很吃惊。

It would be a tragedy if anything went wrong. 如果出了什么事就是一个悲剧。

3.用于if it were not for… / if it hadn’t been for…(若不是因为)。如：

If it hadn’t been for the freezing wind they could have reached the injured man in time. 要不是有刺骨的寒风，他们就及时赶到伤员身边了。

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