除了從屬連接詞、疑問(wèn)代詞、疑問(wèn)副詞、縮合連接代詞和縮合連接副詞外,感嘆詞what、how也可以引導(dǎo)一個(gè)名詞性從句,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.
你給出的這句句子就是一個(gè)以感嘆詞how引導(dǎo)的賓語(yǔ)從句,如果沒(méi)有was,這句賓語(yǔ)從句里就沒(méi)有謂語(yǔ)了（how difficult是表語(yǔ),it是形式主語(yǔ),to drill through the hard rock為真正的主語(yǔ)）.這句話的意思是：“如果他們知道鉆透這塊堅(jiān)硬的巖石是多么困難的話,他們就會(huì)喪失信心了.”這是一個(gè)非真實(shí)條件句,所以用了虛擬語(yǔ)氣.希望我能幫助你解疑釋惑.