这是宾语从句的缩略句型,很常见. 常用于know, teach, learn, expect, think 动词后.
以疑问词+to do的结构出现.
如: I don't know how to do it.
相当于: I don't know how I should do it.
所以这句话也相当于:The mother didn't know who should be to blame for the broken glass as it happened while she was out.
WHO 在这个宾语从句中充当主语. sb be to blame.这是个句型. 不能用whom