it 可作形式主语或形式宾语，代替真正的主语或宾语（不定式、动名词、从句）。
要点讲解
- it 作形式主语代替不定式：It is important to learn English well. 学好英语很重要。
- it 作形式主语代替动名词：It's no use crying over spilt milk. 覆水难收。
- it 作形式主语代替从句：It is said that he will come. 据说他要来。
- it 作形式宾语：find/think/make + it + 形容词 + to do：I find it easy to learn English. 我发现学英语容易。
- 常用结构：It takes sb. time to do sth. 花费某人时间做某事：It took me two hours to finish the work. 我花了两个小时完成这项工作。
- It's time to do / for sth. 该做某事了：It's time for bed. 该睡觉了。
- It's + adj. + for sb. to do（描述事物）与 It's + adj. + of sb. to do（描述人）：It's kind of you to help me. 你帮我真好。
- It seems/appears that... 似乎……：It seems that he is happy. 他似乎很高兴。
- It's one's turn to do sth. 轮到某人做某事：It's your turn to clean the blackboard. 轮到你擦黑板了。
经典例句
- It is necessary for us to protect the environment.
我们有必要保护环境。 - I think it important to keep our promise.
我认为遵守承诺很重要。 - It is said that the meeting has been put off.
据说会议被推迟了。
易错提醒
- 形式主语 it 的真正主语是后面的不定式/动名词/从句；形式宾语 it 用于 find/think/make + it + adj. + to do。