it作替代词的用法
　　 用来替代前面提到的同一事物时,被替代的名词可以是可数名词,也可以是不可数名词。例如:
　　1)We are studying in a school with trees all around it。(it替代可数名词school)
　　2)-Have you still kept your old furniture?
　　-No,I have sold it。
  (it替代不可数名词furniture)
　　 可以替代英语句式中的不定式(短语)、动名词(短语)或从句,在句中作形式主语或形式宾语。例如:
　　1)It is a great surprise to meet you here。(It替代不定式短语to meet you here)
　　2)I think it is no good parents doing anything for their children。
  (it替代动名词的复合结构parents doing anything for短语)
　　3)We found it impossible that they could complete the task in two hours。(it替代that they could。
  。。从句)
 可以替代前面的某一名词或整个句子的内容。这时,可用that替换,也可以改为非限制性定语从句。有时it可以前置替代,用以代替后面整个句子的内容。这时,不可用that替换。例如:
　　1)He said he had never met her,and it is not true。
  (it替代前面整个句子He said。。。)
　　2)I hate it when people talk with their mouths full。(it代替后面整个句子when people talk with。。。)(NMET'98)
that作替代词的用法
　　 at用来替代前面提到的特指的事物,所代替的名词可以是可数名词,也可以是不可数名词。
  代替可数名词可以用the one替换。但其前面不可用任何限定词修饰。例如:
　　1)The population of China is larger than that of Japan。(that代替不可数名词population,不可用the one替换)
　　2)The headteacher in your class is younger and more active than that in theirs。
  (that代替可数名词headteacher,可用the one替换)
　　 at在并列分句中,可以替代前面某一个名词或整个句子内容,这时,可用it替换,也可改为非限制性定语从句。例如:
　　1)She heard a terrible noise,and that brought her heart into her mouth。
  (that替代前面的名词noise)
　　2)They said they had discussed the problem,and that (=it) was impossible。(that替代前面整个并列分句They said。。
  。)
　　 at在定语从句中,可作关系代词,代替某个先行词(人或物)。例如:
　　1)The lady that came to our class is from Australia。(that指代先行词lady)
　　2)A clock is a machine that tells people the time。
  (that指代先行词machine)
。