Luckily, my teacher took her job more seriously than I did, and drummed some basic rules into the heads of her students. I'm not saying I always do it right - I tend to sprinkle in many more commas than most people would find appropriate - but I have the advantage of knowing that there are rules.
The first rule is easy.
If you see an -ing word, look out for a comma.
Some sentences with -ing words are too short for a comma, of course. Like... "I should be writing."
In a longer sentence with an -ing word, there should be a comma.
"When signing up for the class, Maria thought she'd have to re-work her schedule."
"I ran towards the fire, waving my arms in the air."
The comma isn't always right by the -ing word, but it's there.
The second rule will probably stick in my head well after I grow old and forget my own name.
And, Or, Nor, For, But, So, Yet.
The word "and" is tricky, because it doesn't always have a comma, but it often does. The other ones should always have a comma.
The rules about "and" are:
- When it's used to separate an element in a series (three or more things) it should have a comma. This is sometimes called the "Oxford Comma."
- When "and" is used as a conjunction, I.E. to connect two independent clauses, it should come with a comma.
The third rule is about subordinating conjunctions.
The what and the what? A subordinating conjunction is a word that connects a main clause to a subordinate clause. Oh, don't look so scared, it's not as complicated as it sounds. These are words like:
Because, Before, After, Although, Unless, Until, When, As, Since, While, Which
There are more of them, but once you get the feeling for it, you don't really have to think about it any more.
Example: "Maria wrote the blog about commas, because she was so tired of people skipping them."
The word "because" ties the two parts of the sentence together. Note that the comma goes before the word because, not after it.
One more rule. =)
Use a comma with parenthetical elements.
What does that mean? Easy. "Maria's cell phone, which she mostly uses to check her e-mail, fell to the floor."
Besides these basic rules, commas are used to avoid confusion.
Outside the lawn was cluttered with fallen leaves.
Outside, the lawn was cluttered with fallen leaves.
Another good example that has been going around the Internet is:
"Let's eat Grandma."
"Let's eat, Grandma."
Piece of cake! =D