Teaching English: Must Versus Have to

So, I’m an assistant English teacher.  That’s my job.  In fact, more specifically my job is to be a native speaker of English to provide my students with exposure and the chance to interact with a real L1 (First Language) English speaker.  It also means I occasionally run into difficulties that the Japanese teachers don’t anticipate.  When you learn a 2nd language in school, at least in Japan, it’s mostly done by teaching a series of grammatical rules and drilling vocabulary which you can use to fill in the spaces of those rules.  Unfortunately the basis for those rules is often suspect or at least not really firmly rule-like.  Grammar should really be considered a set of guidelines, since people are always mangling them into newly invented monsters.  The monsters lose their hanging chads and polish off their rough edges over the course of time and use, but they’ll eventually change again anyway.