Many people seem unsure how to use colons, however, the principles underlying their use are quite simple. Below is an explanation of the ‘rules’ of their use in Standard English.
In addition to using this punctuation mark when indicating time, as in 4:30am, the colon has three uses in English prose. In general, all four uses signal a grammatical break greater than a comma, introducing clauses that expand or clarify the previous main clause.
1 Making exemplifications and signalling illustrations
In this form the a second clause supports the first, or rather the full meaning of the sentence needs the subsequent exemplification or illustration:
Some animals are exceptional fast: over fifty metres greyhounds can out run the average family car.
Carl Marx was a philanderer: many of his illegitimate children were employed in his Engels’ cotton factories (reference).
This form can be filled, as in:
Many of Marx’s Illegitimate children were employed in Engels’ cotton factories (reference): he was a philanderer.
2. Signalling an illustrative list
This use of the colon can be useful in academic writing when wishing to make complicated lists, separating constituent elements of the list with semi-colons (information on the use of semicolons in extended lists will follow in Section 4 below).
There are four main researchers involved in this study: Professor Susan Jones, JohnHopkinsUniversity; Professor James Smith, University of Illinois; Dr Timothy Taylor, BelfastQueensUniversity; and Dr Samuel Khan, BelfastQueensUniversity.
And this example, adapted from the historian Anthony Smith’s (2008: 12) book The Cultural Foundations of Nations:
For most modernists, the nation is characterized by: a well-defined territory; a unified legal system and common legal institutions; participation in the social life and politics of the nation; a mass public culture disseminated by means of a public, standardized, mass education system; collective autonomy institutionalized in a sovereign territorial state; membership of the nation in an “inter-national” system of the community of nations.
This method of displaying a complex list also lends itself to bullet points; in fact, Smith’s original text has all the elements of the list after the colon as six distinct, numbered points.
3. Introducing an extended quotation/extract
This is particularly common in academic writing. Use the colon when the quotation starts at the beginning of a sentence.
Garton, Montgomery and Tolson go on to suggest:
Particular ideological assumptions and narrative scenarios occupy a place of dominance within this forum, to the extent that their pervasive solidity as forms of common sense is very difficult to challenge. (1991: 116)
Some extended extracts may start mid-sentence and grammatically follow on from your own text, in which case a comma or no punctuation mark might work, as long as this is consistent with the use of commas indicated above.
The colon presents a writer with different forms of sentence construction that just cannot be achieved with commas alone. For that reason they are well worth learning.