The colon is one of the trickier pieces of punctuation to use. Most students avoid them entirely, even up to GCSE and A-level. However, sometimes students want to express ideas in a way that essentially requires a colon. If they don’t know how to use it, you end up with constructions like this:
It was a dream for a boy my age, freedom, one word to describe what I felt. ✖
The student is trying to do something a little bit grammatically ambitious, and they lack the punctuation tools to express it properly. As a result, the sentence is not correct. It works better as follows:
It was a dream for a boy my age. There was one word to describe what I felt: freedom. ✔
How to use the colon
Apart from introducing direct speech or quotations, there are two main situations in which you’d use the colon.
1. Writing a list
The smoothie contained the following ingredients: glue, chicken feathers and banana ✔
2. Writing a list of one.
This is a trickier, more sophisticated use of the colon which is more likely to impress examiners and/or allow to express more complex thoughts.
Aunt Maud took immediate action: she called the police. ✔
Rule 1: Start with a main clause
The fragment before the colon must be a main clause. A main clause is a phrase that can stand on its own – i.e. you could happily place a full stop after it and it would still make sense. If you’d like to get a little more technical, a main clause contains both a verb (a ‘doing’ word) and a subject (the person or thing who is performing the verb). In the case above, the verb is “contained” and the subject is “smoothie”.
The fragment after the colon does not need to be a main clause at all, and in fact it usually isn't. If you are using the colon for a list, you will simply have a list of items after the colon, as above ("glue, chicken feathers and banana").
Rule 2: The phrase before the colon must pose a question, which is answered after the colon.
When we say "the smoothie contained several ingredients" the reader wonders what those ingredients are. In the words of the Oxford Guide to Style, "A colon fulfils the same function as words such as namely, as, for example, for instance, because, as follows..." Consider this example:
Mary broke her leg: she went to the hospital. ✖
Although "Mary broke her leg" is a main clause (subject: Mary; verb: broke), it does not actually pose a question. We are simply told that she has broken her leg, and we're not explicitly invited to wonder about what happens to her, even though we might be somewhat curious. This is therefore an incorrect use of the colon.
Here is a correct use of the colon.
There was only one solution: blackmail. ✔
It's critical to mention the thing that you plan to reveal after the colon. Before the colon we mention "one solution", and after the colon we clarify that this one solution is, in fact, "blackmail".
Here are some more correct uses:
Humans had destroyed that which was most crucial to them: their air supply. ✔
We knew who would win the fight: Mayweather. ✔
The eggs had become sentient quickly and they had a target: me. ✔
Here are some incorrect uses of the colon.
Susan wanted to know why I hadn't arrived yet: my car had broken down. ✖
The koalas were restless: the kangaroos watched them nervously. ✖
Tom had won the race: he had only started training two weeks previously. ✖
Julie enjoyed skiing: she went every winter. ✖
In each of these, we're introducing a new idea after the colon, which makes the whole sentence incorrect. These sentences would be better linked with a semi-colon. A great, fun guide to the semi colon can be found here.
If that all seems to make sense, I have plenty of exercises to let you test your understanding.
1. These are some excellent multiple choice questions about the colon: click here for link.
2. This is an great exercise which shows how to use the colon to make your sentences more forceful: click here for link.
3. Another quiz here.
4. Decide which of the following sentences are correct. The answers can be found here.
- There are two choices at this time: run away or fight.
- We knew who would win the game: the Eagles
- The badgers were furious: never before had their burrow been invaded.
- He wanted to see two cities in France: Paris and Nice.
- She kept repeating: “I really want that car!”
- Here are three states that begin with M: Michigan, Mississippi and Maine.
- You can come pick me up now since: I am feeling much better.
- Never forget this point: think before you speak.
- This house has everything I need: two bedrooms, a backyard and a garage.
- This house has everything I need: I only wish it had a balcony.
- The town reminded me of my childhood vacations: both were on the beach.
- My favourite genres of movie are: drama, science fiction and mystery.
- This was first said by Shakespeare: “To thine own self be true.”
- I bought a lot of meat at the store: bacon, turkey, chicken and tuna.
- The new boss has many nice traits: friendly, outgoing and fair.
- These are my favourite colours: purple, turquoise, pink and yellow.
- This is my next: problem how will I get the horse into the trailer?
- The classroom was neatly in order: the chairs were arranged, the whiteboard was clean and the floors were clean.
- The purpose of these lessons is not just to pass the exams: you should also become better at learning.
- To sum up: work hard and stay out of trouble.
- The goal here is a challenging one: to excel at using punctuation correctly.
Some of these examples have been taken from yourdictionary.com
 Ritter, RM. New Hart's Rules, The Handbook of Style for Writers and Editors. (Oxford University Press, 2005)
Manser, M.; Stephen Curtis. The Penguin's Writer's Manual. (Penguin, 2004)