Six Grammar Mistakes That Drive Me Crazy

Everyone's heard of the self-proclaimed "Grammar Nazis". While I cannot claim to feel my teeth grind every time I see or hear a grammar error (heck, I know my own writing is not perfect), there are a few writing mistakes that drive me nuts. I've seen some of these in self-published works, and they were common in my undergraduate writing courses.

1. Their, There, and They're. 
• Their indicates possession.  
• He is their dog.
• There can indicate a place (real or abstract) or indicate the existence of something with the verb "be".   
• Please don't leave your shoes there.  
•  There are some apples on the table.  
• There is no one here. 
• They're is a contraction meaning they are and can only be used as a subject and verb.  
•  They're coming for dinner tonight.  
• Don't let me forget they're meeting us at the movies.
2. Commas. It seems like when people are in doubt, they use a comma.  Please use commas in the following instances and avoid them otherwise.
•  To separate items in a series.
• Jan, Max, and Phillip went to the game.
• Before a conjunction to connect two independent clauses.
• I drove to her house, but she wasn't there.
• To separate introductory clauses.
• When he got to work, he realized he forgot to lock his doors.
• NOTE: flipping the clauses would eliminate the need for a comma: "He realized he forgot to lock his doors when he got to work".
• After introductory words.
• Yes, I will pick her up at eight.
•  See Purdue Online Writing Lab for further instructions and examples. Others will thank you!
3. Then and Than. 
• Then is associated with time.   
• We ate dinner, then we saw a movie.
• Than provides comparison.   
• She is taller than me. 
4. Its and It's.
• It's is a contraction meaning it is.   
• It's my birthday.   
• Its indicates possession. 
• Its fur sure looks dirty.
5.  Punctuation and Quotation Marks.
• Commas and periods go inside quotation marks unless they are followed by citations.
• Colons and semi-colons go outside quotation marks.
• Question marks and exclamations are more tricky.  If they apply to the quotation only, they go inside the quotation marks.  If they apply to the whole sentence, they go outside the quotation marks.  I know this is one I always have to look up for clarification.
• See Purdue Online Writing Lab for detailed information on quotation marks in general.
6.  The Apostrophe.
• Just please review these guidelines for using apostrophes to create plurals or possessives. 
• Or check out this comic: How To Use An Apostrophe. 

If you're looking for a little humor on the topic, visit The Oatmeal's "Ten Words You Need to Stop Misspelling."

At times I need reminders for commas and quotations, so it's important to have resource handy in case you just don't know proper use. I highly recommend the Purdue Online Writing Lab for any questions relating to citation and grammar. Visit for a quick review whenever you are in doubt or just need a refresher.

Are there any common mistakes you see others making?  What mistakes drive you nuts?