Yes, we all know this one. Pretty simple except for the Oxford comma debate. And Oxford comma is the commas that goes before "and" in the end of a list. Some people leave it out. Some times that is okay, and some times it's downright catastrophic. Here's the thing, using it is rarely wrong, so why not use it? If not this could happen.
2. For places and dates
Yes, we know this one too, but I bet you sometimes forget your second comma. The year or state after a date or town is seen as a parenthetical--anything you can read around where the sentence still makes sense is a parenthetical. Here are some examples:
- September 11th, 2001, will be remembered as our generation's Pearl Harbor.
- She's from Orlando, Florida, which is my favorite place to visit.
3. To join sentences (only if they are joined properly with FANBOYS)
Grammar books tell you about dependent clauses, but to simply this, let's just say sentences. You can join them together via comma, but you need help. Without a helping hand, you create a comma splice which actually can confuse your reader. It is one of those mistakes that is notably a mark of unedited writing. I'm not an agent, but guessing one of these puppies in a query letter is tantamount to a cardinal sin.
- Comma splice She hated strawberries, we wondered why her mother made her a strawberry birthday cake.
- Corrected She hated strawberries, so we wondered why her mother made her a strawberry birthday cake.
Adding that helping hand, FANBOYS (For And Nor But Or Yet So) corrects this issue. There are other ways to fix this as well--create two sentences, use a semicolon, make one sentence no longer one by stripping it of a subject or verb, or making it no longer one sentence by adding a word that makes it dependent (like adding Because to the front of the first bullet).
The key to avoiding comma splices is to realize what makes a sentence--subject, verb, object, and makes sense on its own. Another trick is the age old "pencil test" which I've renamed the "cursor test." You put a pencil/cursor over your comma and look left and then right. If only one side is a complete sentence, you're good. If both sides are full sentences (without FANBOYS), it's a splice. Make one of the above mentioned moves to correct it.