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

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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
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

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
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

it 是形式主语,指代它后面的动词不定式,正常语序为:to drill through the hard rock is difficult,是由how引起的宾语俗语,在这里,作为if they knew 的宾语,语意为:如果他们知道钻透这块坚硬的岩石是多么的困难的话,他们将去失去信心。

it是形式主语。

how difficult (表语)it(形式主语) was(系动词) to drill through the hard rock(真正主语)

it的存在是为了把真正主语部分放在句末,使句子显得平衡。