The colon (:) and the semicolon (;) both appear very frequently on both the SAT and ACT grammar sections. However, they are not commonly used in everyday writing. As a result, a lot of people aren’t familiar with the rules surrounding them. The good news is that both pieces of punctuation are very straightforward. If you can identify an independent clause (the fancy grammar term for something that can stand alone as a complete sentence), you can quickly and easily pick up the rules for the colon and the semicolon.
The colon “:”
When I ask students about the correct use of the colon, they almost invariably say something about how it sets up a list. This answer is not at all unreasonable. Colons are indeed often followed by some kind of list.
However, for the purposes of the test, this answer focuses on the wrong half of the sentence. We don’t really care what comes after the colon. It could be a list, or it could be something else. All we care about is what comes before the colon. A colon must be preceded by an independent clause. Again, this is just a fancy, grammatical way to say that what comes before the colon needs to be a complete thought that could stand alone as its own sentence.
Let’s look at the following two sentences.
For lunch I had: a bagel, a yogurt, and a banana.
I had a great lunch: a bagel, a yogurt, and a banana.
Both sentences use a colon to set up a list. In fact, they set up the very same list. However, all we really care about is what comes before the colon. To test which sentence is punctuated correctly, read all the way up to the colon and stop.
For lunch I had
I had a great lunch
What comes before the colon should be a complete sentence on its own. In this case, sentence 2 fits the bill. Sentence 1 is a frequent trap answer on the ACT English. To fix it, we would just delete the colon. You don’t need any punctuation between “for lunch I had” and the list.
The Semicolon “;”
Semicolons are probably even easier to deal with than colons. For the purposes of the ACT, a semicolon is completely interchangeable with a period. The more grammatical way to say this is that a semicolon comes between two independent clauses.
It’s that simple. If you want to test if a semicolon has been used correctly, cover it up and read on both sides of it. Each side must be a complete thought on its own. You should be able to replace the semicolon with a period.
A good understanding of semicolons can help you quickly eliminate answer choices. For instance, if two answer choices are identical aside from swapping out a period for a semicolon, both can be immediately eliminated.
If you want to use semicolons in your own writing, it’s good to know that they are meant to punctuate thoughts that are balanced and symmetrical. A semicolon shows that each of the clauses is of equal importance (as opposed to a colon, which subordinates the second clause). But the ACT English won’t test you on a “soft” rule like this. All the test-makers care about is that you understand it must come between two independent clauses.
So, for a very quick recap…
The rules for both the colon and the semicolon can be expressed in terms of clauses.
[independent clause] : [whatever]
[independent clause] ; [independent clause]