It took us two hours to finish the job 的原句就是To finish the job took us 2 hours.
用it充当形式主语是为了避免主语比谓语长.
it is to believe to see是错的,因为it is + 形容词/名词 + to-v/v-ing/that-从句,而这句话的结构不合适.
To see is to believe或Seeing is believing就是不定式或动名词充当主语和表语.