Question

it和that的区别

it 和 that 不是都有指代前文出现过的名词吗，那这两词有什么区别呢？
有人说复数用that 单数用it 但不可数怎么办呢

Answer 1

that，those代指在前文中出现的对等的部分（不是同一个事物，只是对等），that指代可数名词单数及不可数名词，those指代可数名词复数
如：The weather in Beijing is colder than that in Kunming.
The apples in red basket are not as sweet as those in black basket.
而it指前文提到的事物，是同一物
如：My pen was lost ,did you see it?

Answer 2

it 是指代离主体比较近的情况，that 是指比较远的情况。

看例，The is my pen. It is in red. 这是我的笔，它是红色的。说话时，笔就在你眼前。

但如说话时，东西离你较远，你要对第三者特意指出来作说明，就要用that.例如，
Where is your seat?
That is my seat.
说话时指著那座位