A few more guidelines for using punctuation:
The semicolon is a good choice when sentences are clearly related, when they seem to go together, when a period would create a too emphatic stop between sentences. Alas, there's no rule to determine whether sentences are related in a way that makes a semicolon a good choice. Making this decision seems to me a matter of acquired intuition.
The presence of a connecting word or phrase (such as nevertheless, therefore, thus, even so, in contrast) is a good sign that you're in semicolon territory. But longish sentences, even if they're clearly related, are likely to be easier for a reader to take in if they're separated by a period.
One caution: it's easy to overuse the semicolon. As an undergraduate, I often used semicolons indiscriminately; I joined sentences together in long, unwieldy chains; my excitement about tying ideas together carried me away; as you can see in this example, the result is not reader-friendly.
When one or more commas appear within items in a series, semicolons should separate the items:
The menu offered limited choices: egg and bacon; egg, sausage, and bacon; egg and Spam; egg, bacon, and Spam; and egg, bacon, sausage, and Spam.*
The dash is a very useful element of punctuation, as it allows for greater condensation in the presentation of ideas. The dash is appropriate in setting off an element that strongly interrupts the movement of a sentence. For instance:
Emily Dickinson and Walt Whitman — the one oblique and elliptical, the other expansive and declamatory — might be said to have invented modern American poetry.The most important thing to remember about punctuation: it's a matter of conventions, shared agreements that help bring clarity to written communication. If you don't agree this sentence unpunctuated difficult to read can serve as a last attempt to persuade.
Three instruments — clarinet, trumpet, and muted trombone — create the unusual tone colors of Duke Ellington's "Mood Indigo."
If you do agree, that last sentence — unpunctuated, difficult to read — can serve to confirm what you already understand.
How to punctuate a sentence