Some of what I’m going to say here might strike you as wrong. You might be more familiar with AP style, which is commonly used in newspapers, magazines, and some websites. However, book publishing uses The Chicago Manual of Style, and that will account for most of the differences between how you think things ought to be written and what is correct for your manuscript.
- Use the serial (or Oxford) comma. That means using a comma before “and” in a series of things, so you have “apples, pears, and peaches.” That second comma is the serial comma. It’s the norm for books and occasionally helps with clarity. You do not need a comma in a list of two items.
- Comma splices happen when you join two or more sentences together just by commas and not by conjunctions or semicolons, commas do not join sentences. (That was a comma splice right there.) You can instead rewrite it to make it into one sentence or make them each into their own sentences. And don’t use more than one semicolon to join sentences. If you feel the need, make a new sentence. Trust me, if you’re wondering about this one, your sentence is too long.
- When someone is cut off or interrupted, a dash is appropriate. That’s when you should use a dash—like this. It should not have spaces on either side of it and it is actually known as an em dash. Word will create it for you if you type two hyphens and keep typing, or it’s easy to find on the Insert tab. Ellipses imply that someone is trailing off, or has just slowly stopped speaking. Use four if it’s the end of a sentence (period plus three ellipses). Be careful not to overuse either dashes or ellipses.
- Avoid participial phrases as they do not make for sophisticated writing. Participle phrases are the most common modifier to misplace or dangle. So instead of, “Breathing hard through his teeth, he gained control over the pain,” instead you would rephrase to: “As he breathed hard through his teeth, he gained control over the pain.”
- Always start a new paragraph when changing speakers. In dialogue, the first word needs to be capitalized every time, and there must always be ending punctuation. If the dialogue tag is a lead-in, it must always have a comma, such as: She said, “Yes.”
Content/developmental editors won’t actually fix up many grammar issues (just their own pet peeves mostly) but a clean manuscript shows you have some understanding of how the English language works, in all its bizarreness, and that you pay attention to detail. It helps editors and agents better see the forest for the trees.