Themostvaluablefindofallwastheship'slogbook,partsofwhichitwasstillpossibletoread.为什么which后面有一个it而变成partsofwhichcouldstillberead就没有用it了

Question

The most valuable find of all was the ship's log book,parts of which it was still possible to read.为什么which后面有一个it 而变成 parts of which could still be read就没有用it了

Answer 1

parts of which it was still possible to read.为什么which后面有一个it：
=it was still possible to read parts of the ship's log book.
parts of which充当read的宾语.
这个it是形式主语,所以得有it
而变成 parts of which could still be read就没有用it了
=parts of the ship's log book could be read.
这里的parts of which充当主语,所以,直接就加could be了.