life in the twenty-first century is much easier than _______

life in the twenty-first century is much easier than ________.
life is much easier than (it) was in the 20 centry
求两个句子具体分析和对比
life in the twenty-first century is much easier than it used to be

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,而后面用过去时态可以看出,前面指的是“现在的”生活.
比较的内容还是哪个日子“好过”.
两句句子是同意句.
再问： life is much easier than (it) was in the 20 centry it为什么不可以换that
再答： 貌似THAT也是可以的，我明天查查再告诉你答案吧。
再问： 额。谢谢
再答： 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)