In writing, punctuation marks are introduced to aid the smooth flow the thought in a narrative. They are introduced to express a change in thought, to recall something from the past, to introduce a new idea, an inflection or change in tone.
But in this article, the focus will be on semicolons, colons, and dashes.
Semicolons (;)
Use a semicolon to separate two main clauses not joined by a coordinating conjunction:
Those who write clearly have readers; those who write obscurely have commentators.
We can also use a semicolon to separate main clauses joined by a conjunctive adverb (such as however, consequently, otherwise, moreover, nevertheless):
A great many people may think that they are thinking; however, most are merely rearranging their prejudices.
Basically, a semicolon (whether followed by a conjunctive adverb or not) serves to coordinate two main clauses.
Colons (:)
Use a colon to set off a summary or a series after a complete main clause:
My set of provisions for the new term is already determined: 3 boxes of cornflakes, 2 large tins of Milo and Peak Milk, 2 dozen canned Titus sardines, and all such things that I may wish to add to the list.
Notice that a main clause does not have to follow the colon; however, a complete main clause generally should precede it.
Dashes (–)
Use a dash to set off a short summary after a complete main clause:
At the bottom of Pandora’s box lay the final gift – hope.
We may also use a pair of dashes in place of a pair of commas to set off words, phrases, or clauses that interrupt a sentence with additional – but not essential – information:
In the great empires of antiquity – Egypt, Babylon, Assyria, Persia – splendid though they were, freedom was unknown.
Unlike parentheses (which tend to de-emphasize the information contained between them), dashes are more emphatic than commas. And dashes are particularly useful for setting off items in a series that are already separated by commas.
In general though, these three punctuation marks – semicolons, colons, and dashes – are most effective when used sparingly.