As it turned out to be a small party,we____so formally

问题描述:

As it turned out to be a small party,we____so formally
答案是选“need't have dressed up'
为什么不选 did not need dress up ,如果在need后加上to可以选吗?

这里表示的是 本不需要
而且这里的need是情态动词
作实义动词是 加to do