When I teach writing in class, the first thing I do is ask students to write a letter so that I can see what they can do. I analyze all the mistakes in that letter and from that point on, I know what to teach. What you have to understand with this is that I am never trying to completely eliminate their mistakes.
You see, if they make no mistakes, it is probably because they are reproducing the same material that they have already learned and are not stretching themselves. So, I focus on eliminating the punctuation and spelling mistakes, and I am trying to eliminate the grammatical mistakes that they are making now and help them to make more complex mistakes in the future.
When the mistakes they make are the same as a native speaker would make then I know that I have taught them well.
