As a former proofreader for a national magazine, I'm perhaps more aware of literary blunders than your casual reader. But, in these days of difficulty getting published, writers need to be more scrupulous than ever so that an editor won't find her "pet peeve" of author mistakes in your manuscript.
If I were an editor, my Top Ten List of No-nos would be the following because I've seen them too often.
1. Could've - could of. There is no legitimate reason to use "could of." I've seen it in printed books and it apparently stems from the author having missed an English class in grade school. He/she means "could've" which is a contraction of the two words, "could" and "have." Example: "I could've been a contender." "I could have danced all night."
2. It's - its. "It's" with an apostrophe, is a contraction of "it is" or "it has." Example: "It's in your best interest to stop doing that now." "Its," without an apostrophe, is a possessive. Example: "That book, even with its tattered cover, belongs in your library."
3. doctors - apple's. While we're on the subject of apostrophes, please remember that a plural word needs none. Example: "The apples were ripe and the doctors ate them." If you put an apostrophe before the "s" you have turned the word into a possessive. Example: "The doctor's time was limited."
4. Nauseated - nauseous. Nauseated is how you feel when something makes you ill. Example: "I was nauseated by the odor coming from the landfill." Nauseous is an adjective describing the thing that makes you ill. Example: "The landfill gave off a nauseous odor."
5. Try to - try and. Technically, there is no "try and." If your character is going to try to do something, use "try to," not "try and." Example: "I will try to help you." However, if you say "try and," you imply you'll succeed. But what if you don't succeed? You've told a lie.
6. I couldn't care less - I could care less. Once again, the second construction should never be used. After all, if you could care less, then you must care somewhat. But you're trying to say that you care so little, it would be impossible for you to care any less than you do.
7. lose - loose. Look them up in the dictionary. To lose something is to no longer have it. Example: "I don't want to lose the lovely watch you gave me." Something which is "loose" is not lost but of an unstable consistency. Example: "The watch slipped off my wrist because the band was too loose."
8. incidents - incidentses. The latter is not a word. One event is an "incident." Two or more events are "incidents." There is no such word as "incidentses."
9. roll - role. As a noun, a "roll" can be a small pastry. As a verb it means moving or turning. Example: "He let the car roll down the hill." "Role" is a noun which describes a part you might play in a film or in life. Examples: "The role required me to exit the stage." or, "I'm tired of playing the role of your wicked stepmother."
10. titled - entitled. When you give a book a title, you have "titled" it. Example: "I titled my book MASQUERADE." "Entitled" means something is owed or expected. Example: "As the eldest I'm entitled to the largest piece."
11. I hope I don't have to tell you that--unless you're writing dialogue in the voice of an illiterate character--you should never write, "Me and my brother," "Her and I," "we was," or "she don't." But I often see "myself" used instead of "me." Don't try to get fancy. Wrong: "She gave the book to John and myself." Right: "She gave the book to John and me." If John was gone, you'd say "She gave the book to me," wouldn't you?
12. farther - further. "Farther" refers to physical distances. Example: "The house we sought was farther down the road." Use "further" to indicate figurative distances. Example: "We had to look further among the possible suspects in the murder."
13. breath - breathe. "Breath" is a noun. Example: "He took my breath away." Breathe" is a verb. Example: "It's so hot, I can hardly breathe."
Well, I see my Top Ten list turned into a Baker's dozen, but I found it hard to choose fewer than thirteen. Writers seem to invent more bloopers every year. Two wonderful (and inexpensive) books on the subject are MORTAL SYNTAX and GRAMMAR SNOBS ARE GREAT BIG MEANIES, by June Casagrande. Her website, grammarsnobs.com, contains her weekly newspaper column, the "grammar lesson of the week," and her Blog. All contain useful information, written in a lively style. And, for punctuation, I recommend EATS, SHOOTS AND LEAVES by Lynne Truss.
If you have some pet grammar peeves, leave a comment. And may your writing contain no bloopers, blunders or boo-boos.