Do you know what my problem with misplaced commas is? It’s that, while I’m reading text in my head, I can’t help taking a pause where the comma indicates, yet my brain knows that it’s wrong. Sort of the same way I have to ‘read’ the misspelling definately as def-i-NATE-ly. (English-writing wordsmiths of the world: it is spelled definitely. Definitely. Think of the word finite. De-finite-ly.)

Anyway, two incorrect comma usages which are like nails on a blackboard to me are commas placed before verbs and before the word that.

The former is most often a case, I believe, of sentences getting out of control. I think we would all agree that a sentence should read Subject verb. and not Subject, verb. (Think I ran. versus I, ran.) But when you start loading up that subject with a bunch of extra words, things can get pretty confusing by the time you get around to picking a verb. So it’s a case of comma-as-crutch, really. You’re basically saying, ‘Hey, we’re all confused by this sentence. So here’s a comma -– let’s take a breather and come back when we’re ready to continue.’

Incorrect Example: The park bench that I usually preferred, was covered in bird poo today.

That’s a pretty bare-bones example, but I wouldn’t be surprised to see that mistake made. You might also see it happen if the subject contained several nouns.

Incorrect Example: The book, an apple, four chess pieces, and a very lazy cat, were all cluttering the surface of the desk.

It’s like I’ve accidentally started a comma sequence I just can’t stop once I get to cat and move on to the verb (the comma before the and is a separate issue and differs between US and UK conventions).

Obviously, there are times when a comma will correctly fall before the verb in a sentence, such as when it's being used in a clause separate from the main subject and verb. Generally speaking, however, it’s a good idea not to let too many words slip between your subject and verb. If you find that you have to take a breather before you carry on with the verb portion of a sentence, perhaps look instead to see if clarity would be better achieved with a rewrite.

Now –- commas and that. They do not mix. Microsoft Word’s grammar checker has but a few simple goals in life. First, point out every sentence fragment in your document; second, suggest ridiculous rewordings to avoid the passive voice; third, freak out whenever somebody puts a comma before which. So here’s the deal with that versus which.

It’s a matter of restrictive clauses and nonrestrictive clauses. A restrictive clause is one that is essential for distinguishing the noun in question.

Example: I chose the copy that had the least damage to its cover.

The information in a restrictive clause is essential, so it doesn’t use commas. It restricts the noun to being only the one that fits the conditions set by the clause. In the example above, I didn’t choose the copy with the second-to-least amount of damage; I chose the one that had the least. Traditionally, that has been used with restrictive clauses, but the use of which is also commonly accepted these days (so take that, Word).

A nonrestrictive clause provides additional information about a noun -– information that isn’t necessary for identification.

Example: I chose the mint-condition copy, which also happened to be the most expensive of the three.

We use commas with nonrestrictive clauses; it marks them as asides, rather than essential information. Here’s the important part: nonrestrictive clauses always use which -- NOT that. It is never correct to write I chose the mint-condition copy, that also happened to be the most expensive of the three. Because that is used solely with restrictive clauses, you should never (under ordinary circumstances) see a comma precede it.

So the next time you’re scouring your writing for errors, pause any time you see a comma occurring before a verb or the word that. You’ll definitely run into exceptions to these rules, but those commas should serve as red flags, indicating either an out-and-out error or an opportunity to reword for clarity.