I've been teaching at a tutoring centre for a few weeks now, and in that time I've had a variety of students, ranging from age 7 to 17, from naturalised citizens to newly landed immigrants.
Aside from the pains of being reminded of the evils of asian parenting, I've found that a lot of the things I end up teaching about English pertain more to style than grammar. Rather, on the student front, I find that more of the errors come from lack of exposure to good style, which I believe is a product of the times we live in as well, where adjectives seem to replace adverbs, and past tense replaces past participles. Nevermind trying to explain to an 8 year-old child that "I and he went to the store" is grammatically correct. One of my maturer students was preparing for MELAB, a sort of American TOEFL. A lot of the multiple choice questions were clearly more to do with idiomatic English and style than proper grammar. Like the difference between "I've been knowing her for twelve years" and "I've known her for twelve years".
And prepositions are fun to teach any age group. Nothing defines a language quite as well as the way in which it uses its prepositions. We say things in English, but in German, it's on. "Auf Deutsch, bitte" literally means "On German, please". Or the difference between waiting on and waiting for someone.
Since the centre itself is extremely new, there is no precedence or policy on a lot of things. Classroom resources, for example, have been written in American spelling, which I assume is due to the negligent user NOT changing the computer/grammar settings of MS Word to English-Canada over the default English-USA. A lot of the sample writings also contain grievous errors that make me wonder who was in charge of English before I came around. Although I haven't taken on the task of editing all their stuff; I've only been correcting 'em as I get them.
Still, like my old job, it's exciting to see a new establishment boldly make its way forward. I just wish travel weren't so expensive. Ah, money. The oil that lubricates the gears of society.