That's Just The Way It Is

Following up on our discussion from Monday, I'm curious to hear what you all have to say about the particulars of tutoring non-native speakers.  Have you had the experience yet?  How did it go?  What kinds of problems arose, if any?  One thing I found when I tutored was that working with non-native speakers forced me to confront just how much of the language I just-plain-know-and-don't-really-know-why.  Why, if I'm in the lobby of a hotel (an hotel?) where there are 4 elevators, do I say "I got on the elevator" and not "I got on an elevator"?  Have you experienced this?  What did you do?  How do you explain those bits of English that you just know?