... because why not learn from others' mistakes?
Some errors need to go the way of the typewriter.
Et tu, dear writer?
In my time as an editor, I've come to two fundamental conclusions about people's writing:
Everyone needs a unique editing approach to address their writing challenges and elevate their own style. 100 writers will have 100 different voices, and there's no single prescription for how to improve any given piece of writing.
There are a select few errors that most people make, regardless of their own style or proficiency.
It's a funny thing, really: Even the most seasoned writer with a mature voice and expansive lexicon seems to fall into the same grammatical traps as every other writer, at least once in a while. These aren't matters of storytelling, but simple conventions that proofreading aims to correct. They're the sorts of errors you'd expect to see on an earlier draft, and which you'd expect an editor to fix, so I'm not surprised that I keep seeing them. But as long as I'm noticing them, I might as well pass that knowledge along to you. After all, if you can get it right the first time, it's one less problem later.
Note that these are relatively small things--matters of mechanics or conventions that are arguably beholden to a style guide somewhere. But they would get fixed in editing nonetheless, so at the end of the day, they still need correcting.
Without further fuss or fanfare, I give you my wholly unofficially and anecdotal "most common proofreading corrections."
The Corrections
Let's just look at how these things should be implemented correctly, so you can compare them against your own practice. Please note that I use U.S. English conventions for the following examples.
Punctuating dialogue
For dialogue that's appended with a dialogue tag (e.g., "he said" or "she exclaimed"), use a comma to end the dialogue's statement: "Hippos are surprisingly dangerous," she said.
For dialogue without a tag, including when followed by an action, use end punctuation: "Hippos are surprisingly dangerous." She pointed to the hippo.
For dialogue that's interrupted by narration:
If the narration interrupts a single sentence of dialogue, use commas. Capitalize accordingly as one comprehensive sentence: "Hippos," she said, pointing at the nearby hippo, "are surprisingly dangerous."
If the narration splits two sentences of dialogue, treat the second sentence of dialogue as a new sentence: "Hippos are surprisingly dangerous," she said, pointing at the nearby hippo. "Be careful."
Whenever possible, follow conventional punctuation rules within dialogue. Some writers opt to break punctuation rules for character voice, such as with comma splices. In most instances, this will simply convey awkward punctuation more than a specific voice/tone, and it will probably get fixed by an editor.
Missing commas
When you join two independent clauses (i.e., complete statements) with a coordinating conjunction (e.g., "and," "but," "or"), use a comma with the conjunction: I like pie, and I like cake. I like pie, but I also like cake.
When you use an introductory word, phrase, or clause in a sentence, follow with a comma: Understandably, some people don't like cake. In this house, we finish our cake. Because I like cake, I eat it often.
Use commas to offset non-restrictive clauses within a sentence (i.e., additional information that doesn't change the sentence's essential meaning): My three children, who are voracious eaters, all enjoy cake.
Em dashes
Ideally, an em dash (—) shouldn't be interchangeable with a period or ellipsis. Try to use it sparingly so as not to distract readers. Conventionally, it's used for the following purposes:
Use an em dash to indicate an intentional break in a sentence's structure or grammar: For my birthday, I asked for a horse—a tall order, admittedly.
Use em dashes to enclose clarifying statements: Last week, I visited my grandfather—my last remaining grandparent—for a nice, relaxing picnic. Note: In this usage, the dashes are arguably interchangeable with commas or parentheses. I recommend using them when a sentence already has multiple commas or when you want to really emphasize the information.
Use an em dash to indicate an abruptly interrupted thought, such as a someone being cut off in dialogue: "I'm not trying to argue with you, it's just that—"
An em dash (—) is different from an en dash (–) and a hyphen (-). Make sure you're using the correct mark, and note that most style guides call for no spaces on either side of it.
Ellipses
In academic writing, an ellipsis (. . .) indicates the omission of information.
In narration or dialogue, an ellipsis indicates trailing off.
Some writers use ellipses often to express characters' uncertainty or lingering thoughts. Try to use them sparingly so as not to litter your dialogue with it and potentially distract readers. Like an exclamation point, it should carry tonal weight, which is cheapened if overused.
Don't begin dialogue with an ellipsis.
Style guides have different formatting rules for ellipses. The Chicago Manual of Style, which is what many publishers use (and which formats well on e-books), calls for 3 periods with spaces between each character: . . .
If that seems like a lot of rules, I beg to differ! This is but the tip of the proofreading iceberg. And yet, it encompasses a disproportionately large number of edits across writers of all ages and persuasions, so I implore you to keep them in mind.
Happy writing!