interogative sentence

Hello,

The construction of an interogative sentence according to a grammar book is like

Quetion word +helping verb (auxi. verb) +?

But sometimes I have seen a few interogative sentences without a helping verb like

Who broke the window?

What happened ........?

I am bit confused about the construction of an interogative sentence start with a question word.

Could anybody here clear my doubt and provide the rules for making an interogative sentence using a question word.