I've spent the last several days preparing a 2-day workshop for software salespeople on writing better emails and other business documents. Two days isn't a lot of time to teach writing improvement, and I always make sure the supervisors who hire me understand that I can present a lot of information and suggestions but that each employee must practice what he or she doesn't know in order to get better. I can guarantee them an entertaining and informative two days, but I can't guarantee better writing. You can lead a horse to the comma....
One of the biggest stumbling blocks in teaching writing improvement is that few people under 40 have any knowledge of grammar or parts of speech. Or rather they have only an internal grammar and not an external one. The internal grammar is the collection of rules and usage that we each develop as infants and children. We're born with this capacity and it gets filled up with our first language,the one we learn from our parents or other caregivers. It's our automatic language and we use it without thinking about it, a lot like we drive a car without thinking about it.
Having an external grammar as well, explicit knowledge of the rules and terminology of the language, is helpful for repairs and stylistics (just like knowing the names of car parts and how the engine works may come in handy if you want to repair your car yourself). If you're a writer by profession or craft, you will want to learn these rules and terminology. (A good guide is a handy book/workbook called The Least You Should Know about English. It's recently been rereleased in a hideously expensive version but old copies are just as good and cheap on amazon.) It's really a must. But other people use writing all the time and don't have a lot of interest in those technicalities.
My challenge tomorrow will be teaching some basics to people who are not writers by trade or inclination but who must improve their writing skills as part of their job. They may not come to the class with much enthusiasm or inclination so I'll have to pull out all my tricks to help them learn where their subjects and verbs are.