the fire was so big that firefighters found hard to get,空格处填it,解释为作形式宾语

题目

the fire was so big that firefighters found hard to get,空格处填it,解释为作形式宾语
我想问一下为什么不能用that,不能把它看作宾语从句吗

答案

the fire was so big that firefighters found it hard to getit是形式宾语,真正宾语是to get ...it宾语, hard宾补,to get...是真正宾语.that是不能充当形式宾语的.如果改为宾语从句则应为the fire was so big that ...