In my essay editing work, I make a lot of corrections where people mix up the use of the comma, semicolon and colon. So let’s take the time to examine the uses of each of these. The reason why these are mixed up is that sometimes a semicolon can be substituted for a comma, and people just get semicolon and colon mixed up because they are both used less often and have “colon” in the name.
The Comma, for a brief pause
Although the comma is generally thought of as a brief pause in the sentence, in fact it is more of a brief pause in the logic of a sentence.
The first place we use a comma is before coordinating conjunctions (and, but, or) that join independent clauses. That means if the clauses both have a subject and verb, and can stand alone as sentences, then they are independent. You can join them with a comma and a conjunction. (Without a coordinating conjunction, it’s a run-on sentence.)
She went to the baseball game, and I went to the opera.
We use a comma after an introductory adverbial clause.
During the rain, I went to the beach because it was not crowded.
We use a comma to set off a non-restrictive clause (usually introduced by “which” and does not “restrict” the preceding noun phrase to a specific individual example).
Purchasing books in university, which is a painful experience, taught me to be a bargain hunter.
Use a comma to separate items in a series
Do you include a comma before the last item in the series? The answer is generally, yes. This troubling comma is called the “Oxford comma” or “serial comma.” Some people are taught that it is required. Others are taught that it is forbidden. The APA style guide wants you to include it. Newspaper style guides leave it off, unless it is necessary for meanning. A famous example of where it is needed could be the following.
A famous example of where it is needed could be the following. In today’s election, I’m voting for Donald Trump, a windbag and a serial liar. Without a serial comma, the sentence makes it look like I’m casting one vote and the comma introduces an appositive or detail about the person I’m voting for. With a serial comma — In today’s election, I’m voting for Donald Trump, a windbag, and a serial liar — it’s clearer that these are three of my ballot choices in a list.
Use a semicolon to form parallel construction
Up above, I mentioned that without a coordinating conjunction, a sentence could be a run-on sentence. But we can fix a run-on sentence without adding a coordinating conjunction by changing the comma into a semicolon. This is my favorite use for a semicolon. Notice that a sentence with parallel construction has a subject and verb in both halves of the sentence. On spring break I went to Hawaii; the snorkeling was excellent. In this kind of sentence, a comma and semicolon cannot be interchanged.
Like a comma, a semicolon can also be used to separate items in a list
But in a list, sometimes we need to use a semicolon because a comma won’t do. In most lists, we use a comma to separate items in a series. But what do we do when we have comma within the items in the series? That’s when a semicolon can come in handy. Sometimes these are lists with long items. In my tour of Europe I visited the Louvre, Paris’s famous museum; Big Ben, the clock on the Thames; and the Leaning Tower of Pisa. I often see a semicolon used incorrectly when the items in the series are long, but don’t contain commas.
Use a colon to expand or explain, or to introduce items in a series
Many people use a semicolon incorrectly to introduce a list. This is the job for a colon.
I’m going to travel to:
A colon can also be used to introduce a clause that expands or explains. Sometimes this can overlap with a semicolon, which introduces a parallel clause that might also explain. However, notice that the semicolon must be followed by a clause with a subject and verb.
I love sports: skydiving, polo, and tennis.
I love sports; skydiving, polo, and tennis are my favorites.
So, the colon, comma and semicolon have similar jobs: they separate things. A colon separates the introduction from items in a list, or from an explanation. A semicolon separates halves of a sentence in a parallel construction. A comma separates items in a list (unless the items include commas of their own) or it separates clauses.
Got any questions? Don’t hesitate to ask a grammar question.