if they knew how difficult it was to drill through the hard rock,they would lose heart.这里

题目

if they knew how difficult it was to drill through the hard rock,they would lose heart.这里
这里it后面为什么加was

答案

除了从属连接词、疑问代词、疑问副词、缩合连接代词和缩合连接副词外,感叹词what、how也可以引导一个名词性从句,如：
She told him what a brave boy he was.
He confessed with regret how foolish he had been.
She didn't know what a crime he had committed.
It's incredible how fast he can run.
You can imagine what difficulties I had with my children.
I told her how late I was.
你给出的这句句子就是一个以感叹词how引导的宾语从句,如果没有was,这句宾语从句里就没有谓语了（how difficult是表语,it是形式主语,to drill through the hard rock为真正的主语）.这句话的意思是：“如果他们知道钻透这块坚硬的岩石是多么困难的话,他们就会丧失信心了.”这是一个非真实条件句,所以用了虚拟语气.希望我能帮助你解疑释惑.