it の用法:形式主語
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英語には、文末重心 [end-weight]という語順の規則があります。文末重心とは、情報量が多いもの・長い句や節を文末に移動させるというものです。主語が重い場合にはそれを文末に移動させますが、その代わりとして主語の位置には「形式主語」と呼ばれる it を置くことになります。形式主語を使わない文は、非常に不恰好でアンバランスに聞こえます。

■真主語に to 原形を置く場合
It is dangerous for children to smoke.
It would be difficult for you to understand this book.
It is impossible to master French in a month.

It is necessary that you should go to her as soon as possible.
It is evident that he hates his brother.
It is true that he won the first prize.

* It is と that が省略される場合もあります:
No wonder / Small wonder he got married to Mary.
= (It is) no wonder / small wonder (that) he got married to Mary.
「彼がメアリーと結婚したのは不思議ではない/大して驚くことではない 」

It doesn't matter who finds the truth.
It is a mystery why he killed himself.
It matters little when you will finish the job.

■真主語にwhether ( or not ) 節を置く場合
It does not interest me whetehr you succeed ot not.
It is up to you whether you go or stay.
It is doubtful whether he will be able to come here.

■真主語に if 節を置く場合 (whether を使う方が一般的)
It is unknown if she will return to school.
It is not clear if he will join our party.

* この形の構文に使われる主な形容詞は以下のように感情的な形容詞が多いのが特徴です
no use = no good = useless「役に立たない」「無駄な」
tough, hard 「辛い」
foolish「ばかばかしい」, annoying「厄介な」
lovely, wonderful, etc

It is no use crying over spilt milk.
It is useless arguing with him.
It is annoying walking in the rain.