第一句：21世纪的生活比以前曾经的生活好过多了.这个句子里面,it是指代life.比较的内容就是哪个日子“好过”.第二句：（现在的）生活比20世纪的生活好过多了.it 也是指代life.从句子的前面用一般现在时态,而后面用过...life is much easier than (it) was in the 20 centry it为什么不可以换that貌似THAT也是可以的，我明天查查再告诉你答案吧。额。谢谢我今天查了语法书。从语法书上看，IT,THAT在指代前述事物时，基本是可以互换的。IT和THAT都可以指代前面出现的一个单词，词组，或者句子。eg. The climate of Thailand is much warmer than that (=the climate)of Korea.需要注意的是，THAT指代的只能是名词的单数。如果指代的名词是复数，要用those， eg. The customs here are different from those (=the customs) in Japan.这个句子就不能用IT来代替了。因为it指单数。it , that 在指代前面词组或者句子时的例子：eg. Don't let your children play with matches or lighters. It/That is dangerous. He tried to help her, but that/it was of no use. 特定用法和结构下，IT和THAT是不能互换的。我把所有情况都列出来了：A:关于 It ：1.强调句：It was/is...that... eg: It was Paul that/who broke the window. \ 去掉强调就是Paul broke the window. It was that 去掉后，句子是通的。 2. It is + adj + to do. It 是形式主语。eg: It is wrong to speak ill of her.3.形式宾语。eg:Don't you find it a waste of time watching TV too much?4.it 做非人称动词主语：It happen/look/seem/appear/occur+that从句。 eg: It appears that he will be elected.(他像是会当选的样子）B 关于 that: 特定用法比it 少得多。主要就是引导从句。 有时虽然出现在特定用法中，但是个人认为也是引导了从句比如：so+adj+that...另外说到THAT和THIS。1. that是前述的内容，THIS是后述的内容。eg: Talking to Helen, I‘ll say this: she seems no harm to you.\ 谈起海伦，我要说的话是这样：她对你好像并无恶意。2.This 是前文中的后者，THAT是前文中的前者。Play is as important as work; this(=work) gives us energy, that(=play) gives us rest。希望以上信息对你有用。我都是从书上抄下来的哦，加上了自己的归纳。不是从哪个网上COPY来的。让人开心的是，其实我自己也是复习带学习了一下:o)