The semicolon is a simple piece of punctuation, much easier to work with than the comma because it follows fairly clear rules. If you learn the two simple rules explained here, you'll rarely go wrong. It has two main uses, which are both easy to identify. You use it to connect two independent clauses together into one sentence, and you use it as a super-comma. You can use it in a few other unusual situations, but they come up rarely, and there are other ways to handle those situations. Learn them if you want to be perfect, but if you learn to recognize the two primary uses, you will do fine as semicolon user.
To Connect Two Independent Clauses
Independent clauses are series of words that could stand alone as complete sentences. When you have two otherwise complete sentences that you want to connect to form one long sentence, use a semicolon between them.
Example: This could be a complete sentence; this could be another one.
If you put a comma where that semicolon is, you will have committed a "comma splice," which is a very nasty grammar error indeed. Sometimes, the second clause doesn't really look like a complete sentence, so you must watch closely.
Example: Twelve workers started the project; only five remain.
There is, however, one exception that can cause you a problem. You don't use a semicolon to connect two complete sentences if there's a conjunction between the clauses (and, but, etc.). In that case, use a comma.
Example: This could be a complete sentence, and this could be another one.
Adding that single word, the conjunction "and," means that you must change that semicolon into a comma.
To Serve as a Super-comma
When you have a series of three or more items that normally would be separated by commas except that each individual item already has a comma in it, you use the semicolon between items.
Example: We visited Pago Pago, Western Samoa; Curitiba, Brazil; and St. George, Utah.
Example: The trio's birthdays are November 10, 1946; December 7, 1947; and October 31, 1950.
Example: Her favorite players are Steve Young, a quarterback; Jason Buck, a defensive end; and Ty Detmer, another quarterback.
As in the examples above, citing places, dates, and people's names with descriptions, are three very common situations where you'll see the super-comma usage.
A few relatively infrequent situations also call for a semicolon. When you list three or more items tacked onto the end of a complete sentence preceded by a connector word such as "that is," "for example," or "for instance," you may use either a comma or a semicolon. Either of the following two example sentences is correct.
Example: Be sure to watch out for grammar errors; for instance, comma splices, run-on sentences, and dangling modifiers.
Example: Be sure to watch out for grammar errors, for instance, comma splices, run-on sentences, and dangling modifiers.
You may also use a semicolon to connect two otherwise complete sentences even if they are connected by a conjunction, if the first sentence already has one or more commas in it. It's optional, but may on occasion make the sentence more understandable.
Example: When I eat alone, I leave a mess; but that's not the worst of it.
Both the minor uses noted above are optional, so if you can remember only the first two situations above, you'll never go wrong by putting a semicolon where it doesn't belong.