He did not know _ to do about it.

A
因为 to do 缺宾语,所以选疑问代词what,疑问副词how 不可以做宾语为什么要宾语呢？主谓后面接一个how的从句不可以吗？可以呀！ 从句也是作宾语的呀，英语中叫“宾语从句”He did not know（主谓语） 【what】_ to do about it 【带疑问词的不定式短语作宾语】.He did not know（主谓语） 【what】_ he should do about it 【what引导的从句作宾语----宾语从句】.