On my Evernote, I have a collection of grammar anomalies – rule “transgressions” and things that don’t fit into the simplified patterns we teach students. How sad is that?
As teachers, some of us are fascinated by grammar. At least, we have a professional interest in being curious about all the rules and exceptions. Some students are too – I’m sure you can think of some. (They’re the ones that want an in-depth explanation of when we can and can’t use certain verbs in continuous tenses, and whether there is a rule to cover this, and they’ll sit through these explanations making copious notes.)
I suspect that most students aren’t fascinated by grammar. Of course, they want to be accurate, but more than this, they want to get their messages across in English. This is one reason why the clarification stage of lessons should ideally be concise, and focused on more genuinely problematic areas. There’s no need to walk through the negative and interrogative forms of every structure in great depth, even if your CELTA tutor insisted on it.
On CELTA courses, I always encourage the trainees to research the target language in some depth – typically the analysis is more thorough than what will be touched on in the lesson. A knowledgeable teacher is a good thing, but so is light-touch clarification – because grammar is a means to an end.