请教 IT 做形式主语（宾语）或代词的区别

achange

请问如何判定 it 在句中是做形式主语（宾语）？还是代词？

比如：
# Prep2008-256：
Scientists have dated sharp-edged flakes of stone found in the fine-grained sediments of a dry riverbed in the Afar region of Ethiopia to between 2.52 and 2.60 million years ago, pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.

A.when it is known that humans made
B.at which it is known that humans had made
C.at which humans are known to have made
D.that humans are known to be making
E.of humans who were known to make
答案：C

A,B的错误之一在于：“此选项表达的意思是A fact is known at the earliest date，逻辑含义错误。”
因此我得到的结论是，It 在这道题里做的是代词，错误的指代了the fact。
而我以为它是形式主语，真正主语是 humans。为什么不对呢？

而在#PREP2008-241的补充说明中有举例：
it可以替代句中的不定式或从句等, 充当形式主语或形式宾语。one与that均无此用法。
It is known to everybody that the moon travels around the earth once every month.

所以彻底晕了。。。。

flywer

不客气~~一起加油

achange

恩，明白了。晕在形式主语代不定式/从句上了。多谢！

flywer

你的理解没有问题的，it在这里就是作的逻辑主语啊，这里所说的fact就是that从句后面所引导的从句的内容。（是解释将它简称为fact了，因为that从句后面的内容是一个事实嘛）
请注意，it在作形式主语的时候一般是只能指代不定式或是句子，所以it在这个句子中作形式主语指代的是后面的that从句，而不是指代humans(humans还是复数呀，怎么能用it作形式主语指代呢)。而按照这样理解，将that后面的内容代到前面it所在的位置，就是：pushing back by more than 150,000 years the earliest date when that humans made stone tools is known .   无论是时态还是句意就都被曲解啦（这样一来句子的意思就变成了：人类制造工具的事实在150,000多年前就被知道了）。

不知道我说清楚没~仅是我个人的看法哈  借鉴参考~