1、不定式作逻辑主语
如：It is very kind of you to give me a hand in time.
你能及时帮助我，真是太好了。
2、从句作逻辑主语
如：It is so happened that the tickets were sold out.
碰巧票卖完了。
3、动名词短语作逻辑主语
如：It is no use reasoning with him.
和他讲道理是没有用的。
4、作形式主语的代词只能用it ，不能用that，this等词。
如：It is easier to lose friends than to make friends.
交朋友难而失去朋友比较容易。
拓展：5、it是形式主语，真正的主语是V-ing形式
It is considered no good reciting without understanding.
不理解的背诵被认为是没有好处的。
形式主语的4种句型
代词it常常用作形式主语来代替句子中的不定式短语或者动名词短语。此时it为形式主语，被代替的部分为真正的主语。这种情况可归纳为四种句型：
1、It is +形容词+不定式短语
例如：lt is important to learn
English well.
2、It is+名词+不定式短语
例如：It is our duty to protect the earth.
3、It is no use/good +动名词短语
例如：It is no use /good arguing with him.
4、It is no use/good +不定式短语
例如：It is no use/good to do such things.
