How to Use Semicolons
A memory trick and helpful chart.
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Semicolons Emphasize Relatedness
Another reason to use a semicolon is to draw attention to how related your two clauses are. The semicolon in our example sentence highlights that the reason you can't go out tonight is that you have a big test tomorrow. You wouldn't write, “English is my fifth period class; I can't go out tonight,” because those two main clauses have nothing to do with each other. I can't think of a single reason why English being fifth period would mean you can't go out tonight.
Semicolons and Coordinating Conjunctions
You should never use a semicolon and a coordinating conjunction such as “and,” “so,” and “but” to join two main clauses; that's the job of a comma. If you want to use a coordinating conjunction you'd write it like this:
I have a big test tomorrow, so I can't go out tonight.
Nevertheless, there are a couple of instances where it's OK to use a semicolon with a coordinating conjunction.
Semicolons Can Join Complex Clauses with a Coordinating Conjunction
First, if you have a long sentence with multiple independent clauses, and some of those clauses contain internal punctuation such as a comma, you can use a semicolon with a coordinating conjunction to make the separation between clauses more clear. Here's an example:
If you want me to go out tonight, you need to help me with my homework first; and if you say no, I'll know that you don't really care about going out.
Because each half of that long sentence has a conditional clause that must contain a comma, it's OK to use a semicolon before the “and” that separates those two parts. You could make them two sentences, but you don't have to; and because they are so closely related, it makes a lot of sense to have them be together separated by the semicolon. The “and” after the semicolon is actually optional in this case, but I think it adds to the flow of the sentence.