Here’s a quick explanation of correct semicolon usage:
For the great majority of situations, there are only two instances when semicolons are appropriate.
1. To separate items in a series when one or more items in the series have their own internal punctuation, or when one or more items contain a lot of words. Examples:
a) At the store, he purchased a toy truck, which cost $5; a toy rabbit, which cost $3; and a toy piano, which cost $20.
b) At the store, he purchased the toy truck that he had seen advertised the night before on television; the toy rabbit that his daughter had told him about 20 times in the past week; and the toy piano that he had been thinking about since last February when he saw it at a friend’s home.
In (b), the choice between using commas or semicolons is up to you. Either would be OK. Just use semicolons if you’re afraid the reader will get lost in all the words and lose track of the fact that you’re still reciting a series. A semicolon tends to be a more pronounced demarcation between lengthy items in a series, whether there’s internal punctuation or not.
2. Use a semicolon to end one independent clause, and introduce another independent clause, when the two clauses are closely related and you’re not connecting them with a conjunction such as “and,” “or” or “but.” In these cases, a period will almost always work, too. (An independent clause is simply a clause that could stand alone as a sentence if not preceded by a semicolon or conjunction.)
Never use a semicolon before a clause that couldn’t stand alone as a sentence. Use a comma in those cases.
Never use a comma before a clause that could stand alone as a sentence (when no conjunction is being used); that creates a run-on sentence.
If a conjunction is being used to link two independent clauses, then do NOT use a semicolon also. It’s either one or the other but not both.
The word “however” is not a conjunction, and should not be used to connect two independent clauses unless used along with a semicolon or period.