it 在句中作形式主语或形式宾语的替代用法

　　1. it作形式主语可以代替句中的动词不定式、动词-ing分词或一个句子。

　　It is important for us to learn English well. (it替代不定式to learn English)

　　It is no use making such an apology to us Chinese. ( It 替代动词-ing分词 making such an apology)

　　It is happy for her parents that she is well now. (it 替代句子that she is well now)

　　2. it作形式宾语可以代替句中的动词不定式，此时句中的谓语事由某些特殊动词 (如：findthinkfeel等)充当的。

　　I find it impossible to finish so much work in such a short time. (it 替代不定式to finish so much work)

　　that this在句中的替代用法

　　1、 that

　　A: I’m going to wear my new jeans to her party.

　　B: You can’t do that. (that替代to wear my new jeans to her party这一事情)

　　2. this

　　this 在介绍某人或某物时，指的是靠近自己的一方。

　　A: Hi, Mr. Smith, This is my friend, Bill.

　　B: Nice to meet you, Bill.

　　C: Nice to meet you, Mr. Smith.

　　This 在电话用语中指的是自己。

　　A: Hello, may I speak to Miss Li?

　　B: This is Miss Li speaking. (我就是李女士)