You'll notice throughout this lesson that some compound sentences have a comma as well as a connector, and some just have the connector. A pure grammar guide might give you these rules:
- Add a comma whenever you are connecting two simple sentences:
"I'd shot the other one just below the chest, and he was dead too."
- Don't add a comma if the actor and verb helpers are being carried through:
"A bale of hay has come loose and spilt itself all over."
If you just want to leave it at that for your own writing, that's fine.
But we've already seen a whole bunch of snippets that don't follow those rules. So what's going on?
Commas create separation. Leaving out a comma that should be there can make two things feel more closely related:
"Brian turned the wheel slightly and the plane immediately banked to the right."
"Brian turned the wheel slightly, and the plane immediately banked to the right."
Adding in commas slows the reader down. The commas in this snippet help the reader understand what's happening (or not happening), and add weightiness to the situation:
"But people weren't rushing to their next class, or playing around, or spinning the locks on their lockers."
"But people weren't rushing to their next class or playing around or spinning the locks on their lockers."
It's important, though, if you're playing around with adding or removing commas, that you don't make your sentence harder to read or understand. If you're not sure, ask a friend to sense-check for you.