it 和that 都可以指代前文的某一事物,但是怎么使用它们呢

好的,我给你例句：
Mr Wang is very kind.He gives us lots of help.
==Mr.Wang gives us a lot of help,and THAT is very kind of him.
that指代前面提到的Mr.Wang gives us a lot of help
==It is very kind of Mr.Wang to give us a lot of help.
It(形式主语) is very kind （表语）of Mr.Wang to give us a lot of help（从of到最后都是真正主语）.------it的使用是为了使句子变得平衡.

that和it差别是 that，those代指在前文中出现的对等的部分（不是同一个事物3.it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

它们，他们和她们都可以用 they
比如：I love dogs, they are very cute.
希望这能回答你的问题 :D

it是指前面提到的东西
that 代替不可数名词，如
Your shirt is very nice . Where did you buy it ?
The weather Qiongqing is hotter than that in Beijing in summmer ?