The mother didn't know who (to blame) for the broken glass as it happened.

该句中，to blame 可以用 was to blame 代替吗？或者也可以用 should be blamed代替？

Who is to blame = Who should be blamed ? 正确吗？

疑问词+to do不定式是间接特殊疑问句的简略形式。即省略与主句相同的主语和情态动词，将情态动词后的不带to的不定式改为为带to的不定式。所以，原来的间接问句形式如下：

The mother didn't know who she should blame for the broken glass as it happened.

又如：

He didn't know what to say.

He didn't know what he should/could say.

因此，虽然原句中的to blame可以用was to blame或should be blamed替换，但原句中的who to blame却不是这个二个句子简化来的。

The mother didn't know who she should blame for the broken glass as it happened.

The mother didn't know who should be blamed for the broken glass as it happened.

The mother didn't know who was to blame for the broken glass as it happened.

以上三句都是正确的。但原句是第一句的简略，而不是第二和第三句的简略。因为疑问词作主语，不能只将情态动词+do改为to do。 例如下句为错误：

I don't know who to come. (错误）

I don't know who will come. (正确）