Editing Tip of the Day: Hey Look, a Comma

Commas are tricky for every writer. We all have our problems with them, whether it's overuse, underuse, or dropping them in random places where they just don't belong. Today's tip is about how to join two independent clauses with a comma.
A clause is a chunk of a sentence that contains a subject and a verb (or a predicate, if you want to get technical). Clauses can be extremely short and simple (she left, I am, etc), or they can be longer and more complex. An independent clause is something that can stand by itself as a simple sentence and still make sense. (I went to the store. I took a nap. I read a book. My life is boring.)
So what does that have to do with commas? Good question.
As authors, we often like to stick two short sentences together to make a longer, more interesting one. Instead of that string of short, boring sentences I wrote above, I could link some of them with conjunctions. And, but, or, nor, for, yet, so.
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IF you link two independent clauses together with a conjunction, you usually need a comma before the conjunction.
Which looks like this:
I went to the store, and then I took a nap. I want to read, but I have work to do. I could finish my work, or I could watch my favorite show.
However, if the clauses both have the same subject and one of them is shorter than five words, you can skip the comma.
Like this:
I went to the store and I bought some milk.
But if the clauses have two different subjects, then you still need a comma, even if one is shorter than five words.
And that would look like this:
Mary puked, and Tom gagged at the stench.

To be completely, shockingly honest, I don't remember all these rules word-for-word off the top of my head. They're great to know, and to have cheat sheets for, especially if you struggle with a particular usage. And sometimes publishers and editors will have different preferences for comma usage. So write down the rules you struggle with the most. Then keep rhythm in mind as you punctuate, and remember that a comma typically marks a small pause. And if you still struggle with your commas, it's okay. That's what editors are for!