Student or Learner
Factories must to refine oil befor it can be used as fuel.
This is a sentence from the book, Essential words for toefl.
May you please explain to me that why there is to after the modal verb "must" and what is its function?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.