Mistake #1: Writers don't place a comma between independent clauses separated with a conjunction. Independent clauses are clauses that can stand on their own as sentences, e.g., "He took the 405 freeway to work, and he exited at the Getty Museum." Because both "He took the 405 freeway to work" and "he exited at the Getty Museum" are independent clauses—meaning they can stand alone as sentences, you must, must, place a comma before the conjunction, "and." This is probably the biggest, most common mistake I find in manuscripts and books. Don't make it. It's a very easy punctuation rule to remember.
Mistake #2: Writers place commas between independent clauses and dependent clauses. This is probably the second most common mistake I see. A dependent clause is one that cannot stand on its own as a sentence. Let's take the above example, and change it just a little: "He took the 405 freeway to work and exited at the Getty Museum." I took the second "he" out. That makes the clause after "and" a dependent clause, because "exited at the Getty Museum" cannot stand alone as a sentence. It is dependent upon the first clause to be understood; thus, no comma should precede the "and."
Of course, there are other places you need—and don't need—commas, but this isn't meant to be a comprehensive study of the comma. If in doubt, look up comma placement in The Chicago Manual of Style or other style manual.
Mistake #3: Writers don't know their homonyms. In just the last few weeks alone, I've seen characters who were unphased, waiving to people, and peaking out windows. The writer's spellchecker should have alerted her to the fact that "unphased" isn't even a word. She meant "unfazed." To waive means to relinquish, to set aside. The word this author wanted was "waving." And a peak is the highest point of something; one peeks, not peaks, out a window.
Please, unless you are 100 percent sure you are using the right homonym, look it up. The wrong choice could have your characters doing some pretty strange things!
Mistake #4: Writers rely on their spellcheckers. This is a big no-no. If ewe think you're spellchecker will fined awl yore miss steaks, your wrong. That sentence went through my spellchecker just fine, and there are no less than eight errors in it ("ewe" should be "you"; "you're" should be "your"; "fined" should be "find"; "awl" should be "all"; "yore" should be "your"; "miss" and "steaks" should be "mistakes": and finally, "your" should be "you're"). Homonym spelling errors are the most common type of spelling error I find. Do not rely on your spellchecker. It will let you down every time.
Mistake #5: Writers who make errors in syntax. For example, look at this sentence: "I saw a deer driving to work today." Uh, no—you didn't, unless there are some very talented deer in your neighborhood! The correct sentence structure is, "I saw a deer while driving to work today," or, "While driving to work today, I saw a deer." Please, don't put the deer in the driver's seat!
Here's another example: "If your toddler won't drink milk, warm it in the microwave for a few moments." Warm what in the microwave? You've got a choice of antecedents here. Heaven help the toddler if you make the wrong choice! The correct structure would read, "If your toddler won't drink milk, warm the milk in the microwave for a few moments."
Of course, if you and I were having a conversation, we'd probably understand each other if we made these syntax errors. But you can't count on that when people are reading your words. Make sure you have them in the correct order so your meaning cannot be misconstrued.
I cannot list every error I run across while editing manuscripts. To do so would fill a book. But if you watch for these top five mistakes in your writing, your manuscript will be a lot more polished, and you can be more confident about submitting it to your publisher.