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.