Writers do themselves a great favor by learning to understand punctuation conceptually and fundamentally, as follows:
A comma is a separator. Therefore, when you use one you should identify why the material is worthy of separation. Common reasons include that you used a transition word or phrase that creates a natural pause; you wrote a lengthy, complex sentence with multiple subjects and verbs; and that you supplied a list of three or more related items or phrases in a row.
A colon is an arrow pointing forward. It tells the reader that new information, which is promised by the wording before it, is about to arrive. The colon is especially handy for introducing an announced piece of evidence, a focused example, or a list. Contrary to popular belief, the colon can be used to point us forward to a single word or to an entire sentence, as in the old George Carlin joke: “Weather forecast for tonight: dark.”
A semicolon is a mark of co-dependency. This mark is so often confused with the colon that their distinction bears mention: “The colon is two dots; the semicolon is a comma below a dot.” As the explanation demonstrates, the semicolon is usually used to join phrases or sentences having grammatical equivalency, and it emphasizes that the joined parts are related, even co-dependent, in context.
A dash redefines what was just said. It is a powerful way to make an important aside or to tack on an additional comment of consequence—a comment that redefines. When typing the dash, be certain that you don’t type a hyphen, but two hyphens in a row or a long bar.
