I've heard it said that language is not only prescriptive, it's descriptive too. The propensity to get swallowed up into the formalities of grammar can actually become counter productive in my humble opinion. A grasp of grammar is a good thing. If one intends on working as an interpreter, okay then, get down to the business of learning the formal rules of the language.
What if, however, one is learning for the pleasure of learning. How does a teacher teach the basics of grammar, and show the student how to use the language to communicate in day-to-day conversations with native speakers? A person's face can light up when they learn to say something in another language. Teach the student not the agenda.
Good luck with your teaching goals!