第一句：21世紀(jì)的生活比以前曾經(jīng)的生活好過(guò)多了.這個(gè)句子里面,it是指代life.比較的內(nèi)容就是哪個(gè)日子“好過(guò)”.第二句：（現(xiàn)在的）生活比20世紀(jì)的生活好過(guò)多了.it 也是指代life.從句子的前面用一般現(xiàn)在時(shí)態(tài),而后面用過(guò)...life is much easier than (it) was in the 20 centry it為什么不可以換that貌似THAT也是可以的，我明天查查再告訴你答案吧。額。謝謝我今天查了語(yǔ)法書(shū)。從語(yǔ)法書(shū)上看，IT,THAT在指代前述事物時(shí)，基本是可以互換的。IT和THAT都可以指代前面出現(xiàn)的一個(gè)單詞，詞組，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指代的只能是名詞的單數(shù)。如果指代的名詞是復(fù)數(shù)，要用those， eg. The customs here are different from those (=the customs) in Japan.這個(gè)句子就不能用IT來(lái)代替了。因?yàn)閕t指單數(shù)。it , that 在指代前面詞組或者句子時(shí)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. 特定用法和結(jié)構(gòu)下，IT和THAT是不能互換的。我把所有情況都列出來(lái)了：A:關(guān)于 It ：1.強(qiáng)調(diào)句：It was/is...that... eg: It was Paul that/who broke the window. \ 去掉強(qiáng)調(diào)就是Paul broke the window. It was that 去掉后，句子是通的。 2. It is + adj + to do. It 是形式主語(yǔ)。eg: It is wrong to speak ill of her.3.形式賓語(yǔ)。eg:Don't you find it a waste of time watching TV too much?4.it 做非人稱動(dòng)詞主語(yǔ)：It happen/look/seem/appear/occur+that從句。 eg: It appears that he will be elected.(他像是會(huì)當(dāng)選的樣子）B 關(guān)于 that: 特定用法比it 少得多。主要就是引導(dǎo)從句。 有時(shí)雖然出現(xiàn)在特定用法中，但是個(gè)人認(rèn)為也是引導(dǎo)了從句比如：so+adj+that...另外說(shuō)到THAT和THIS。1. that是前述的內(nèi)容，THIS是后述的內(nèi)容。eg: Talking to Helen, I‘ll say this: she seems no harm to you.\ 談起海倫，我要說(shuō)的話是這樣：她對(duì)你好像并無(wú)惡意。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。希望以上信息對(duì)你有用。我都是從書(shū)上抄下來(lái)的哦，加上了自己的歸納。不是從哪個(gè)網(wǎng)上COPY來(lái)的。讓人開(kāi)心的是，其實(shí)我自己也是復(fù)習(xí)帶學(xué)習(xí)了一下:o)