当 “it” 作为形式主语时，它可以替代以下不同类型的从句。
It作主语从句形式主语
- it＋be＋形容词＋that从句：
- It is important that we recycle.（回收利用很重要。）
- It is essential that we conserve water during this drought.（在这场干旱期间，节约用水至关重要。）
- It is crucial that you arrive on time for the meeting.（你准时参加会议至关重要。）
- It is surprising that she didn’t know about the party.（她竟然不知道派对的事情，真是令人惊讶。）
- It＋be＋名词（词组）＋that从句：
- It is a fact that the Earth revolves around the Sun.（地球绕太阳转是事实。）
- It is common knowledge that smoking is harmful to health.（众所周知，吸烟对健康有害。）
- It is a mystery why the cat suddenly disappeared.（猫突然消失的原因令人费解。）
- It remains uncertain whether the concert will be rescheduled.（音乐会是否会重新安排尚不确定。）
- lt＋be＋过去分词＋that从句：
- It is believed that she has already left for the airport.（据信她已经去机场了。）
- It is believed that the ancient city was abandoned due to natural disasters.（据信这座古城因自然灾害而被遗弃。）
- It is rumored that the treasure lies hidden in the old castle.（传言宝藏隐藏在那座古老的城堡里。）
- It is known that he was awarded the Nobel Prize for his groundbreaking research.（众所周知，他因开创性的研究获得了诺贝尔奖。）
- It + seems/happens/ appears/doesn’t matter/makes no difference＋that从句：
- It seems that the meeting has been postponed.（看来会议已经延期了。）
- It doesn’t matter that the train is delayed; we have plenty of time.（火车晚点并不重要，我们有足够的时间。）
- It makes no difference whether we eat out or cook at home tonight.（今晚我们出去吃还是在家做饭都无所谓。）
- It作what, who, how, when, where, why等引导的主语从句形式主语：
- It is unclear what caused the power outage.（目前尚不清楚导致停电的原因。）
- It is not clear who will attend to this matter. 目前尚不清楚谁将处理此事。
- It hit her how exciting the news was. 她突然意识到这个消息是多么令人兴奋。
- It doesn’t matter too much to me where you go. 你去哪里对我来说并不重要。
it作不定式形式主语
- It is + 形容词 to do sth.：
- It is difficult for him to give up smoking.（戒烟对于他而言很困难。）
- It is unwise to give the children whatever they want. 给孩子想要的任何东西都是不明智的。
- It is + 名词 + to do sth.
- It is a pity to miss the party.（错过派对真是遗憾。）
- It is not a good habit to stay up too late. 熬夜不是一个好习惯。
- It is + 介词+ to do sth.
- It is against the law to steal. 偷窃是违法的。
- it is in my turn to perform on the stage. 轮到我上台表演了。
it作动名词形式主语
- It+be+形容词+动名词
- It was very hardt getting along well with those men. 和那些男人相处非常困难。
- It was nice meeting you. 见到你真高兴!
- It’s foolish behaving like that. 这样的表现是很愚蠢的。
- It+be+名词+动名词
- It is no use crying over spilled milk.（哭泣于已经洒出的牛奶没有用。）
- It is no good arguing with him.（和他争论没有好处。）
- It is no pleasure watching TV anymore.（看电视已经不再有乐趣了。）