只有特殊疑问代词what,which,who 引导宾语从句的同时可以做从句的主语.如：
I don't know what should be done.
Can you tell me which is right?
He told me whom had come to the office.
这些宾语从句有时可以转换成疑问词+不定式：
I don't know what to be done.
不过,当疑问代词在宾语从句中作宾语时,都可以转换成"疑问词+不定式“.如：
I don't know whom I should ask.= I don't know whom to ask.
Can you tell me which I should choose?= Can you tell me which to choose?