I teach in Japan and although my level of Japanese is okay, I never find myself needing to resort to Japanese to explain grammar. A good 4 week TEFL course like the CELTA usually teaches all you need to know to be able to do that. Looking at the grammar in a bunch of books like The Cambridge Grammar of English and Murphy's and choosing the best explanations and borrowing bits of it for your grammar presentations would also help.
In this case, were you using the old "Some for statements, any for negatives and questions" over-simplification? Even with the typical textbook add-on of "but some for requests and offers", it doesn't take students long to find exceptions. The much more general rule, from The English Verb by Michael Lewis, I think, is that any means "zero or not zero" e.g. "Haven't you had any tea??" and some is an amount somewhere between a bit and a lot, e.g. "Would you like some tea?"