if they knew how difficult it was to drill through the hard rock,they would lose heart.这里
问题描述：
if they knew how difficult it was to drill through the hard rock,they would lose heart.这里
为什么要加it,其什么作用
答
形式主语 这里有一个宾语从句how difficult it was to drill through the hard rock 真正的主语是to drill through the hard rock 为了避免头重脚轻所以用了形式主语it