I am not a teacher.
@Freeguy. I think you should be careful about this. Native speakers inevitably make judgements (however much they might like to think they don't) on the way others express themselves.
Coming from the mouth of a native many different factors are taken into consideration. Speaking just for BrE I can assure you that the accents and speech patterns of natives are instantly recognized and assumptions are made. If the person sounds foreign the assumption will be that s/he is making mistakes.
Your aim shouldn't be to strive to make mistakes that natives can get away with easily. The result won't be to blend in but to stand out.