When students first learn to edit, they’re looking for mistakes everywhere. And they find a lot. But one of the things I try to teach from the first day is that not everything has something wrong with it, so they need to know when to leave copy the heck alone. A principle that guides physicians should guide editors as well: First, do no harm. (Interestingly, this exact phrase does not appear in the Hippocratic Oath.)

In other words, if it ain’t broke, don’t fix it. (Or, as an overzealous editor might say, if it isn’t broken, don’t fix it.) This is one of the cardinal rules of editing: Have a good reason for any change you make.

Don’t edit to your peeves. Don’t change something simply because “it sounds bad.” Don’t alter perfectly fine writing because that’s not the way you would have written it.

Do fix the mistakes. Do fill in missing information. Do smooth out rough patches. Do trim words that add nothing. Do explain the complicated. That’s plenty of work to keep an editor busy without messing around in things that don’t need messing.

What editors hate more than letting a big mistake slip through is inserting a mistake themselves. One way that can happen is by mucking about unnecessarily in copy — that’s a typo, a dropped word or a repeated sentence waiting to happen.

It’s often said that good editing is invisible. (Great editing can chop 200 words and no content whatsoever from a piece, but let’s save that for another day.) That’s as it should be. The whole point of editing is for the reader to smoothly understand everything without being distracted by anything. And sometimes good editing means doing nothing at all.