疑问词加动词不定式：
a)构成：what,which,how ,when,where等疑问代词或疑问副词,后面几个一个动词不定式或动词不定式短语.
b)作用：按所作的句子成分来说,在句子中一般可以作主语,宾语和表语.如How to save the child is the most important to us.(作主语)
I don’t know what to do next.（作宾语）
The problem is when to start.(作表语)；
“疑问词+动词不定式”结构可以改写成由该疑问词引导的从句,从句所作的句子成分不变.如I don’t know what to do.= I don’t know what I should do.