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It 句式大盤點
■本站特約作者 劉永科
It 既是代詞，也是引導(dǎo)詞（語法亦叫功能詞），是最簡單也是用法最復(fù)雜的一個詞。作為代詞，it主要用于指代前文提及的某種事物、某種情況，甚至還可以指性別不明確的人；也可以指時間、日期、地點、距離、天氣、溫度、環(huán)境等。作為引導(dǎo)詞，it可以用作形式主語、形式賓語以及引導(dǎo)強調(diào)句型等。關(guān)于it作為代詞的用法，大家都已經(jīng)十分熟悉。本文不再贅述，而主要對it作為功能詞的用法，做一歸納和總結(jié)。
1. It is necessary (for sb.) to do sth. 句式
在該句式中，it是形式主語，動詞不定式是真正主語，不定式的邏輯主語用for sb.。句式中的形容詞通常表示不定式的重要性、必要性、難易程度等基本情況。如 important, necessary, natural, easy, safe, common, normal, hard, difficult, dangerous, unusual, rare, impossible, pleasant等 。例如：
It is very important for us to take part in physical training out of class.
It is too difficult for a three-year-old child to answer such a question.
不可以說：
*We are very important to take part in physical training out of class.
*A three-year-old child is too difficult to answer such a question.
2. It is kind (of sb.) to do sth. 句式
該句式與上述句式十分相似，形容詞往往表示不定式邏輯主語的品質(zhì)或特征屬性，所以用of sb., 不用for sb. 常見的這類形容詞有：good, bad, brave, careless, clever, cruel, foolish, honest, horrible, kind, lazy, modest, naughty, nice, polite, rude, silly, stupid, wise, wrong,等。例如：
It is very kind of you to help me out of trouble.
It was brave of him to jump into the river to rescue the drowning boy.
上述兩句可以轉(zhuǎn)換為：
You are very kind to help me out of trouble.
He was brave to jump into the river to rescue the drowning boy.
3. It is no good (use, pleasure, fun) doing sth. 句式
該句式中，it是形式主語，表語部分是否定意義，形容詞往往表示“益處，用處，樂趣”等含義，真正主語一般用動名詞形式。例如：
It is no good learning English without speaking English.
It is no fun climbing mountains on raining days.
4. It is clear (apparent, obvious, true, probable, likely, possible, impossible, certain…) that句式
該句式中，it 是形式主語，真正的主語是that 引導(dǎo)的主語從句，這是it引導(dǎo)主語從句最常見的一種結(jié)構(gòu)。
It is very clear that he’s round and tall like a tree. = That he’s round and tall like a tree is very clear.
It is very likely that he will not consent.= That he will not consent is likely.
5. It is important (necessary, right, strange, natural...) that句式
該句式跟上一個同屬一種類型。由于主句中的形容詞性質(zhì)不一樣，它們表示事物的重要性、必要性、正確與否以及帶有“出乎意料的語氣”，所以，that 從句中要用虛擬語氣（should + 動詞原形）。建議讓學(xué)生記住用于該句型的形容詞。例如：
It is important that we should learn English well.
It is necessary that he should remember these language rules.
(https://www.cpsenglish.com/people/548)
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