It 句式大盘点
It 既是代词，也是引导词（语法亦叫功能词），是最简单也是用法最复杂的一个词。作为代词，it主要用于指代前文提及的某种事物、某种情况，甚至还可以指性别不明确的人；也可以指时间、日期、地点、距离、天气、温度、环境等。作为引导词，it可以用作形式主语、形式宾语以及引导强调句型等。关于it作为代词的用法，大家都已经十分熟悉。本文不再赘述，而主要对it作为功能词的用法，做一归纳和总结。
1. It is necessary (for sb.) to do sth. 句式
在该句式中，it是形式主语，动词不定式是真正主语，不定式的逻辑主语用for sb.。句式中的形容词通常表示不定式的重要性、必要性、难易程度等基本情况。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.
不可以说：
*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
该句式与上述句式十分相似，形容词往往表示不定式逻辑主语的品质或特征属性，所以用of sb., 不用for sb. 常见的这类形容词有：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.
上述两句可以转换为：
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
该句式中，it是形式主语，表语部分是否定意义，形容词往往表示“益处，用处，乐趣”等含义，真正主语一般用动名词形式。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
该句式中，it 是形式主语，真正的主语是that 引导的主语从句，这是it引导主语从句最常见的一种结构。
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
该句式跟上一个同属一种类型。由于主句中的形容词性质不一样，它们表示事物的重要性、必要性、正确与否以及带有“出乎意料的语气”，所以，that 从句中要用虚拟语气（should + 动词原形）。建议让学生记住用于该句型的形容词。例如：
It is important that we should learn English well.
It is necessary that he should remember these language rules.
