For some reason, colons have a tendency to make people nervous. There’s really no reason for concern, though, because there are only three things you need know in order to use them flawlessly.
Colons can be used in two situations:
1) Before a list
New England consists of six states: Maine, Vermont, New Hampshire, Massachusetts, Rhode Island, and Connecticut.
2) Before an explanation
Correct: I spoke to my supervisor, and this is what she said: I should come in early tomorrow morning in order to make up for the shift that I missed last night.
The first half of the sentence sets up the information after the colon, which explains what the supervisor said.
Let’s look at a slightly more challenging example.
Correct: The Greek goddess Athena was a complex figure: she was a patron of the arts and music, but she also was a warrior who was typically depicted as wearing armor and carrying a shield.
Again, the second half of the sentence expands on the first — it explains why Athena was a complex figure.
Now, there is an additional requirement:
The sentence before a colon must be able to stand on its own as a complete thought.
For instance, in the second example above, the statement The Greek goddess Athena was a complex figure is both a grammatically complete sentence and a statement that makes sense on its own.
In contrast, we cannot say this:
Incorrect: The Greek goddess Athena was: a complex figure.
Even though The Greek goddess Athena was is technically a complete sentence because it contains a subject (The Greek goddess Athena) and a verb (was), it does not make sense as an independent thought.
One more example.
Incorrect: In recent years, forest fires have become: an increasing threat in the United States and abroad.
Because In recent years, forest fires have become does not make sense as an independent statement, it should not come before a colon.
Correct: In recent years, forest fires have become an increasing threat: they burn earlier and later in the year, both in the United States and abroad.
Because In recent years, forest fires have become an increasing threat does make sense on its own, it can come before a colon.