This list is a compilation of the most common errors that I've come across while grading papers in a required writing course for incoming college freshmen. I use this page as a key to comment on their papers; grammatical errors are indicated in the margin by the corresponding rule number. The students can then use this site or a printed version of the rules to learn about the rule and correct their errors. This helps students to realize that many of their mistakes violate a small but significant set of standards. Simultaneously, the use of this list eliminates some of the redundancy involved in the instructor's commenting upon student papers.
Thanks go out to my students of English 1001 at The University of Colorado for their efforts and examples, and to all of the users of these pages who have offered feedback.