When should I (not) use a comma?

This semester, I am teaching ENV 350 (Advanced Environmental Science) here at Drew. The class will have junior and senior Environmental Science students serving as environmental consultants for local small businesses! I see this as their opportunity to formally transition from environmental students to environmental professionals (to the extent they haven’t already done this, of course!).

In that spirit, I see it as my role to give them targeted instruction in areas that could make the difference between them looking immature or like students and looking like professionals. In other words, I have observed that there are “ways” I and other professionals can tell when we are dealing with other professionals versus with students or non-professionals. Not all of these are fair, and they certainly should not be grounds for disrespect, of course! However, I want to make these “markings” clear to my students so they can better ensure they are always treated with the respect and standing they deserve!

One thing I simply cannot unsee is poor command of commas in writing. Think about it: The comma may be English’s most commonly used punctuation mark! Not every sentence ends with a period, but, if one does, it only ever has the one period. However, a single, grammatical sentence may contain a dozen commas! For this reason, to me, a poor understanding of commas and when to (not) use them equates to a poor understanding of how to write, and professionals definitely know how to write! After all, a professional scientist’s job is to communicate useful knowledge to others; poor writing skills fly in the face of that, right?

So, here’s my practical “guide” to proper and professional comma usage, at least insofar as it’s relevant to common professional writing situations. After all, formally, there are something like 17+ comma “rules” in English. Many of those don’t come up often–I would absolutely forgive someone for getting those rare instances “wrong.” I would also totally forgive someone for foregoing optional commas or getting one or two comma rules “wrong” sometimes. It’s when a person seems to use commas all willy-nilly that my “judge-o-meter” starts to spike! Hopefully, this guide will help you not only improve your usage of commas but also help you to “organize” the common rules better in your mind so that their application comes more naturally to you!

First, some things commas cannot (or should not) do:

#1. Glue together two complete, equal thoughts.

Consider this example: “I don’t know much about grammar, I do know a lot about commas.”

Here’s how I think about this: Commas are common because commas are useful, but they are individually weak. They are pauses, not intermissions. They are braces, not casts. They are lane markers, not guard rails. Here, we have two thoughts that could stand entirely alone–if I told you just one or the other, you wouldn’t be necessarily hanging around, waiting for me to say more, right? A comma is not “strong” enough to hold together two independent thoughts like this–it’s a brace when a cast is needed. There are two options to fix this example:
–Use a stronger punctuation mark. Here, either a period or a semicolon could do the trick: “I don’t know much about grammar; I do know a lot about commas.”
–Add a coordinating conjunction. These are “linking” words/phrases that can add some heft to a flimsy comma: “I don’t know much about grammar, but I do know a lot about commas.”

In effect, the second option makes the second thought here a “dependent” thought–it adds to and clarifies the first but isn’t quite equal to it (and wouldn’t be considered necessary to understand the first thought). Commas can and should be used to connect an independent clause to a subordinate clause like this! However, you’d have to decide if marking the second thought as “more optional” than the first is something you’re comfortable with doing.

#2. Separate a subject from its verb.

In English, a clause is a subject + a verb. These things share a strong linkage that no single comma should interrupt. Consider:
“Writing about the proper usage of commas, is pretty challenging.”

Here, we have an example of the always confusing gerund phrase (a verb in -ing form actually functioning as a noun or part of a noun). “Writing about the proper usage of commas” is functioning as both a noun and as the subject of the sentence here. It doesn’t feel (nor look) right to just take a little grammatical nap in between setting up this subject and then explaining what it’s “doing,” right? I don’t usually see this error in simple sentences. More often, it’s an issue in sentences that are long and the subject and verb are put far apart because of intervening information:
“Whether or not you think it’s right for people to judge you for how you use commas, doesn’t mean they won’t.”
Here, the entire thought starting with “whether or not” and ending with “commas” is our subject here–it’s “what our sentence is talking about.” Our verb phrase here (“doesn’t mean”) comes four words from the end! There’s no business having a little speedbump sitting in between it and the subject, so the comma here is improper.

In science writing, the goal is to be as clear as possible. The above example shows that, often, we end up misusing things like commas when our sentences are maybe too complicated or wordy to begin with. Comma misuse is often a symptom of a larger problem! Compare the above sentence to this reworked one that means the same thing:
“People may judge you for your comma use, whether they should or not.”
Having a long, complex subject, putting the verb near the end, having an implied second subject (“people”) hidden at the end…these were choices we made in the first example that we can unchoose by just scrapping our first version and starting over!

#3. Set aside essential information from the rest of the sentence.

Oy. This one is going to be hard to explain. Consider the following example: “All Christmas trees cost extra, that are pre-decorated.” Kind of a wonky example, sorry, but it illustrates my point. I want you to consider the first part to start with: “All Christmas trees cost extra.” Subject + verb = complete thought. Internalize the meaning. Then, tack on the second part. Oh, it’s not all Christmas trees that cost extra…that wouldn’t even make sense, right? I couldn’t actually draw useful meaning from just the first part of this sentence, as it turns out, and yet critical information I needed was in another zone, sectioned off by a comma. If the goal of writing is to convey meaning, the comma doesn’t help us here to actually do that. The word “that” is actually a helpful clue here–we rarely put unnecessary content after the words “that” or “who/m.”

Some instances of this are subtler, though. Consider:
“The Vampire Weekend album, “Father of the Bride,” is folksier than their previous albums.
The middle part here is what’s grammatically called an “appositive.” It is a clarifying or distinguishing phrase for a noun elsewhere in the sentence. Here, it clarifies the question of which album we are talking about. In this sentence, we have cordoned off the appositive with commas, marking is as “in another lane” from the rest of the sentence. Consider: Does the sentence make equal sense with and without the appositive? Nope–I have no idea which album is folksier than others without the appositive! Since that clarification is needed, the commas should be left out.

#4. Separate just two equal subjects, objects, and/or verbs from each other.

For example:
“My friend, and I are both scientists.”
“My wife both cooks, and paints well.”

“I went to the store, and the pharmacy.”

None of those examples should feel right to you. It’s hard to articulate the “rule” here, but these compound nouns/verbs/objects are a “package deal” and yet we’ve split them up here with these unnecessary commas. It’s like tapping on the brakes before going through a green light! That said, again, this is a much more common problem when the two parts we’re talking about are long, such as the infinitive phrases here:
“I went to the store to see if they had any of those peaches that my wife likes, and to see if they were out of flour again.

In all these cases, “and” (as well as other, similar coordinating conjunctions) is often a clue that things are being treated as equal and that a single comma should not come in between them!

NOW, SOME THINGS COMMAS CAN (AND SHOULD) DO:

#1. Bracket/separate a parenthetical (non-essential) element.

As we sorta discussed above, we sometimes add things to sentences that aren’t essential to their meaning. They add something of value, perhaps, but the reader could do without them and still get our point. It makes sense, then, that we could use something like a set of commas to put these elements in their own little “stalls” so the reader could decide what, if anything, to do with them. Consider this example:
“When I was young, in my teenage years, I was a faster typist.”

You don’t really need to know exactly how long ago I was a faster typist to get my meaning here–but I’ve provided this information anyway, in case you were wondering. The commas on either end of this prepositional phrase mark the beginning and end of this little aside/anecdote I’ve thrown in. Think about it: This mentally helps you put these two different pieces of information on different levels in your mind and makes it easier for you to throw out the less crucial info and keep just the stuff that matters to you.

The commas, as they often do, also improve clarity though! Consider the sentence without any commas at all:
“When I was young in my teenage years I was a faster typist.”

Hmm…Does this sentence say the same thing as the previous one? Depends on how you read it! It could…or it could be instead saying that it was specifically in my young teens that I was a fast typist. OR it could be saying I was a faster typist whenever I was “young” in my teens, as opposed to all those times I was “old” in my teens…whatever that means! The commas encourage me, as a reader, to pause and consider what exactly is probably being clarified or modified by this little optional chunk.

Note: If the parenthetical element is at the beginning or end of the sentence, you can use just one comma (or a comma and a sentence-ending punctuation mark) to section it off.

#2. Separate three or more equal items that are in a list.

Well…no one ever said English grammar was super consistent! Earlier, I said that a comma should never separate any two equal things from each other. There are two caveats of sorts to that rule. First, you should use commas to separate any three or more things from each other. Lists can be confusing for the reader, so this provides a bit of order that helps a reader keep track of things, tell when one item ends and another begins, and know when the list is over. See–I just used commas to do that! Elsewhere on my blog, I have a whole guide to parallelism–the process of putting “equal” things into lists in writing. I recommend checking it out for more on this complex subject!

#3. Separate two (or more) equal adjectives modifying the same noun.

And here’s caveat number two. When you are modifying a noun with two equal but distinct adjectives (and one adjective is not really an adverb modifying the other adjective!), separating them with a comma can help the reader to know they need to apply those adjectives to the noun separately. Probably easier to explain with an example:
“Alex is wearing a strong red coat.”

This example illustrates the most common issue with leaving a comma out here. Does this sentence say that the coat is both strong and red, or that the coat’s color is a strong shade of red?? Unless context clues help us out, we’d be guessing. However, when you see the comma used in between “strong” and “red,” you’d know it’s the former meaning and not the latter (as long as the writer understands how to use commas!).

#4. Ending introductory elements (but it’s complicated!).

Consider the following examples:
“Honestly, I don’t know what I’m doing here.”
“Above the sink and behind all the other dishware, you can find some extra glasses.”
“To know what to do at all times, you should read the instructions.”
“Taking my time, I try to come up with a good example.”


Here, we have an adverb, a prepositional phrase, an infinitive phrase, and a gerund phrase, respectively, starting our sentence. Notice that none of these phrases are acting as the subject of the sentence (“you” or “I” is playing that role. Instead, they are modifying or clarifying the rest of the sentence. When you have one of these elements, it is technically optional to set them apart with a comma. That said, it’s a good idea to do it whenever any of the following are true:
–The opening element is long, such that it might be hard for the reader to tell when it has ended (see example 2 above).
–Making the reader pause for effect after the phrase helps convey your meaning better (see example 1 above).
–The modification is “optional” meaning-wise, in which case it’s like setting aside a parenthetical element (see example 4 above).
–The meaning could be unclear if a comma doesn’t step in to sort things out. Here, consider example 3 above again–let’s remove the comma and re-consider the sentence:
“To know what to do at all times you should read the instructions.”
Without a comma as a guide, a reasonable reader might interpret this sentence this way:
“To know what to do [pause] at all times you should read the instructions.”
In which case, you might have some poor undergrads thinking they need to be spending every waking hour (“at all times”) reading the instructions! >P.

TBH, whenever someone tells me that something is “optional” when it comes to grammar, I take that to mean “I should do it unless not doing it is clearly better.” That’s how I feel about commas for introductory elements as well for the Oxford Comma, which I talk about in my blog post on faulty parallelism. Rarely do optional things hurt you, but they can something really help!

So, that’s the highlights! There are other comma rules (such as how to use commas when writing dates), but they don’t come up as often in professional science writing. Still, there are a number of excellent guides out there besides mine if you want to dive deeper (e.g. this one). Let me know what needs to be explored more, and good luck with your commas moving forward!