这里that为名词替代词，而不是指代词。that 替代 the life. 是为了避免重复名词，前面的life 和that 替代的the life 是不同的life 。这就是替代，而不是指代。Life is interesting and I enjoy it.  这里it 为人称代词，指代life. it和life表示的是同一个概念，是一回事。这就是指代，而不是替代。that用来替代特指的单数名词或不可数名词。those用来替代特指的复数名词。one用来替代单数泛指名词，ones用来替代复数泛指名词。one和ones 还可以与this，that，these, those结合替代特指可数名词。

人称代词it只有指代作用，没有替代作用。

this也不能用作名词替代词。

The weather in Beijing is different from that in Shanghai.   that 替代 the weather

Trees in the south are similar to those in the north.   those 替代the trees

That car is better than this one.   this one替代 this car

These cars are better than those ones. t  hose ones 替代 those cars.

These cars are cheap. You should buy one.   one 替代 a car.

I don't like this car. I like the one on the left.   the one 替代 the car.