Put into a Comma after Punctuation-palooza

So you have this bundle of assignments that you got done two weeks ahead of finals because you're an especially responsible student who enjoys a relatively stress-free conclusion to his/her semester. First of all, bite me. Second of all, good for you. Although now you're probably getting anxious; none of your professors accept early assignments because they have "other things to work on" before they receive the torrent of papers and accompanying papercuts that arrive just before holiday break. What to do, what to do.

Here's a crazy idea: go back over them. Now I know it's painful to go back and look at old work (I have a whole folder of assignments on my computer that I refuse to open because I'm sure it'll send me into shock), but you haven't turned in your assignment yet. Basically, you're not quite done yet. That compare/contrast paper you did on the political associations of Fox News and MSNBC? You know, the one you knocked off in a couple hours so you could go see the new Twilight at midnight because your friends are fans of the series but you never saw one before because you read high-brow stuff like David Sedaris and frequent McSweeney's? Welp, you might just want to glance over it one more time.

MSNBC has Rachel Maddow, Andrea Mitchell, and Chris Matthews.

Fox News features Bill O'Reilly, Shepard Smith and Greta Van Susteren.

Look hard. There is a difference between those two sentence, one that goes beyond the content. I'll give you a hint. It's not the periods. Give up? It's the commas. You use them in a series. While in the first series you use one before the "and," you didn't put one before the "and" in the second one. Now, it's my understanding that either is acceptable but here's a word of advice: consistency. You want to keep the stylistic choices you make consistent or someone's going to take them for mistakes as opposed to a decision of style.

Commas are a weakness, you say? Let's change that. Let's make them a strength.

Oh, here's another spot.

Fox News personalities tend to sit on the right-wing of politics; conversely many MSNBC personalities sit on the left-wing.

Astute observation. Bravo on the semi-colon. But you forgot something. It's not the--

Yeah, it's the comma. Or rather, the lack thereof. Any idea where you might jam that comma?

Fox News personalities tend, to sit on the right-wing of politics; conversely many MSNBC personalities sit on the left-wing.

NOPE. Try again.

Fox News personalities tend to sit on the right-wing of politics; conversely, many MSNBC personalities sit on the left-wing.

Huzzah! Right after the transition! Bravo. One more spot.

Keith Olberman was once on MSNBC, but has since left.

Now let's--okay, so you're getting it now. Commas. Solid. But what do you think the issue is with this comma? Still nothing? Try reading after the comma and conjuction:

has since left.

Who has since left? What has since left? What did you do with that subject?! By jamming that comma before the phrase you created a compound sentence out of a sentence and a fragment--a union that should never be.

Now there are some other spots in that paper where you made similar mistakes with commas.

Eliminate them.