[Eric18]

For worse or for better, I've found teaching countable and uncountable nouns among the most challenging tasks - even to advanced students in college. Many students continue to write "homeworks", "informations", and "advices" - all good mistakes - despite several years of studying English.
A large part of the problem is that the rules remain less consistent than the rigid grammar formulas usually taught and memorized- and the language keeps changing. Consumer culture, for instance, is fast changing coffee and tea from the uncountable and adding a countable option as savvy consumers adopt the language of specialists. Blame Starbucks if you want, but the language keeps growing and meeting the needs of users. Of course, it will take the TOEFL test creators a decade or so to acknowledge this change.
The two best concise overviews on this topic that I've found remain Cambridge University's outstanding Grammar in Use series - and the appendix to Academic Writing for Graduate Students by Swales/Feak.
Why do I mention all this background information? Because many ESL instructors often expect their beginning and intermediate students to master this distinction in just a few lessons, and feel frustrated when students continue to make "good mistakes." These Countable/uncountable problems - along with related problems with articles (a, an, the) - often linger and appear almost intractable.
Or so it seems to me.
Good luck with your classes and teaching countable and uncountable nouns!