答：很好的问题.
Today is much hotter than it was yesterday.
It is much hotter than it was yesterday .
1 两个句子 是否都正确?第二句,应该是对的,
主要是,第一个句子,前半句里面的 Today,是否要换成 It 才对?前后对应?
答：第一句将Today改成It,并将时间状语today后置,或将从句中的it was删除.
析：It可以指代天气（the weather).,than引导比较状语从句,常省略和主句中相同的句子成分.
第一句的主句主语是today,则从句中的代词it指代的就是today,逻辑不同.
改法有二：
1.It is much hotter today than (it was hot) yesterday.
2.Today is much hotter than yesterday (was hot).
括号中是省略的句子成分.
2 It is much hotter than it was yesterday .
这里 it was yesterday.( 是否是,相当于省略了 It was ( hot ) yesterday.)
答：是的,前面已经回答.不过最好在主句中加上时间状语today
3 如果我直接说,
It is much hotter than yesterday 行么?yesterday 这里当做名词用?
答:不行,因为主句中的it=the weather,不是“天气”和“昨天”,而是今天的天气和昨天的天气在比较.
Today is much hotter than yesterday 这么来说 行不行?
语法上 说的过去吗?
答：可以.行.