When either you are reading something someone has written or you are writing something yourself, what English composition challenges or "problems" most vex you? Or, to ask the question about English composition in a different way, what are your pet peeves and nagging weaknesses?
As Jesus said, the law was made for man, not man for the law. Either way, though, you have to know what the law is. The one time I taught freshman composition, I had already worked as an editor and been published as a writer. I started by telling the students to write me a letter. Then I sat with those who seemed to have the hardest time, and went through what they had written and edited it for them. Pretty much the way a guy taught me how to do carpentry. The greater your knowledge of the laws of grammar and usage, the better able you are to know when they might get in the way of what you want to say. "I don't feel good" may not be the best English, but it gets across something that "I don't feel well" doesn't. The important thing to remember is that writing is interesting. Once you get the kids interested, it's all pretty smooth sailing. I have been told that all the students in my two classes turned out to write pretty well. If so, it wasn't just because I did my job. It was also because they did their job.