1. it作形式主语句式：it is +adj. + (for/ of sb.) + to do sth.
句中选用for还是of，取决于adj.(形容词)。 如果adj.是描述人的属性的形容词，就要用of；其他要用for。
例如：It is kind of you to help us.
It is important for you to learn English hard.
2. it作形式宾语句式 ： find / think / make + it + adj. + to do/ that从句(宾语从句)
初中阶段只有上面列举的三个动词(find / think / make)才有这个句式
例如：Tom found it difficult to learn Chinese well.