if they knew how difficult it was to drill through the hard rock条件状语从句
how difficult it was to drill through the hard rock宾语从句
it was to drill through the hard rock中的it是形式主语
it was to drill through the hard rock中的to drill through the hard rock是不定式做真正的主语,
they would lose heart是整个句子的主语
要是还有不懂的再问我