这个句子中第一个it为形式主语,第二个it为人称代词,指代the apple. that从句为比较分句中的外置主语从句。此时,可以省略形式主语it ,同时省略that 从句,得到:
The apple is bigger than was previously thought.
was可以进一步省略,得到:
The apple is bigger than previously thought.
如果保留形式主语it则为语法错误:
The apple is bigger than it was previously thought. (为什么不能把it 看做the apple,这样的话 比较分句的it 和主句的the apple 主语一致 ,这句话不就是对的吗?)