I congratulate all writers who submit their work to proofreaders and editors for a final polish before submitting. A review by a professional is especially important for book manuscripts. Yet, more often than not, when pressed as to who was the editor or proofreader, a writer will respond that the manuscript was read by a friend or relative who was an English teacher, majored in English in college, or got As in English while in school. I would bet the vast majority of literary agents and editors would back me up on this statement.
An English teacher or major is not a professional editor nor a proofreader. I was an English major who got As in English. That gave me the natural inclination to seek employment in the publishing industry. My first job was editorial assistant. My first day I was given a sheet with proofreader's marks to learn and a style book, Words Into Type. I spent one year learning the basics of copy editing before becoming an assistant editor, at which point I was assigned a few minor titles to work on under the supervision of another more experienced staff member. I learned how to move a book through the various stages of the publishing process from manuscript to bound book. After another year, I was promoted to associate editor and became responsible for more titles and more in-depth analysis of what it takes to create a successful book in terms of content, organization, ancillary products and marketing. Then I became an editor and my last position at that publishing house (after 5 years) was senior editor and I supervised 16 titles, 2 staff editors and a host of freelance editors.
I'm bringing this up to point out the difference between an editor and an English teacher or someone who is gifted with language. We have the same basic talents, but very different training. There is much more to fine tuning a manuscript than finding spelling and grammatical errors. I have reviewed manuscripts submitted to me that were edited by English teachers. I find errors. It's not that the teachers are not good in the classroom, but they are not trained in print production. I have a two-page checklist of things to review in a manuscript. An English teacher who is editing your work is looking for spelling and grammatical errors, which is last on my list—not unimportant, but it's the final step, not the only one.
An editor's job isn't limited to finding errors; an editor can make suggestions for better organization, presentation and flow. Everyone needs an editor, including editors. Two pairs of eyes are a must [period]. Make your second pair of eyes a professional editor. Yes, it is an expense. Publishing is a business. If you're serious about seeing work published, then investing in a good editor is a cost of doing business.