Parts of which it was still possible to read.这是书上的句子，为什么加it？

it 是形式主语,真正的主语是to read.
形式主语是为了避免句子头重脚轻的现象

不通啊，which与 it 只能留一个
或者是which part of it was still possible to read?

如：把这句话变成陈述句：it was still possible to read sth.
而上述那句只是把sth改了，用which来代替了，自然it就有了。