teaching a foreign language can sometimes be like doing linguistics the wrong way round. grammatical study should be based on sentences that have been produced and understood in actual contexts. we should expect to find some imprecise, changeable conventions, but not any rules. it can be difficult, though, to teach a language with that outlook. a learner will be dispirited if you suggest just taking language as it comes and learning from experience. learners want rules, as they seem to offer security and a starting-point. of course, once a teacher has been foolish enough to give a rule, any awkwardly enterprising student can easily find justifiable exceptions to it though, of course, they will not really be exceptions to anything.