What a mistake (it is) to have refused his invitation.

to have refused his invitation在此句子中是真正的主语,it是形式宾语。将此感叹句改为陈述句是：It is a mistake to have refused his invitation。之所以用感叹句,其目的是此错误之大,遗憾之深。例如：What a pity (it is) to have missed the wonderful film。 再问： 感觉像定语，感觉又像形式宾语。