The Nitpicky English Major's Guide to Grammar :: Where does my comma go?

{logo image found on vintageprintable and altered by me}

Have you heard of the Oxford comma?  I like it.  I use it in lists of three or more items such as books, movies, or magazines.  See what I did there?  There's a comma after books, after movies, and after magazines (the Oxford Comma is the one after "movies").  But I'm not going to talk about the Oxford Comma all day, because it's really boring, even to those of us crazy enough to major in English.  I'll spare you. 

However, the Oxford Comma shows the reality that the use of the comma is often disputed.  One English teacher taught me to use the Oxford comma; another crossed it out in all my papers.  But there are always a few places that you pretty much always need one.  The first is after any sort of prepositional phrase that begins a sentence.  Here are some examples, with the prepositional phrase in bold:

When I go to the bank, I deposit money.  

Under the bridge, there was water.

In between the house, car, and garage, a kitten was asleep.

Now, a preposition does not always denote a prepositional phrase, like this:

Under the bridge was where the water collected.

Here, "under the bridge" works as the subject of the sentence, meaning that it performs the action of the sentence, which is, in this case, "was." "Under the bridge" functions as one noun.  You don't need a comma.  

So how do you ever tell the difference?  You can think about how you would replace the words in question.  If you could replace the group of words with "that," it's probably not a prepositional phrase.  Look at this example again.  The words that may be replaced with that are in bold.

Under the bridge was where the water collected.

That was where the water collected.

Meanwhile, you can see that replacing "under the bridge" with "that" doesn't work in this sentence:

Under the bridge, there was water.

That, there was water.

It doesn't sound or look right, does it?

So when you do have a prepositional phrase (like this little bit right here before the comma in this sentence), you will need a comma after it.  A pretty good list of prepositions can be found here, and if you have a phrase that begins with one, I'd recommend looking at the examples above and evaluating whether or not you need a comma.  

{image found on yourenglishlessons; original source noted in copyright on image}

In summary, you can reasonably determine your comma needs, at least when it comes to prepositional phrases, by answering these questions:

1. Is there a preposition in my sentence?

2. If yes, does it introduce a phrase (such as in something, through something, or because something)?

3. If it does introduce a phrase, where does this phrase end (usually after the "something" in question 2)?

If you answered yes to the first two questions, place the comma at the end of the phrase, as determined by question three.

I'll talk about comma use for interjections and other grammatical functions another time.  This is quite enough for now!