How do I explain to someone whose first language is not English the following concept: If I want to ask you about something, the phrase is "How does it work?" If I am about to explain how something works, I see, "This is how it works."
You can't interchange them, so asking "How it works?" is improper grammar, as is saying "This is how does it work" as an explanation.
Is there a shorter way to sum up this rule?