Semicolons and Colons
Use semicolons to connect independent clauses without a coordinating conjunction. Also use semicolons to connect independent clauses separated by transitional expressions (i.e. conjunctive adverbs6 and transitional phrases7).
A semicolon is another way to avoid a comma splice. Simply place the semicolon between independent clauses. However, make sure the independent clauses you're connecting have something to do with one another. Linking two unrelated sentences with a semicolon is not correct usage.
INCORRECT: I took English this semester; I don't understand my economics class at all.
CORRECTED: I took English this semester; it was really hard.
Semicolons can also be used as an alternative to periods when separating independent clauses with transitional expressions. Commas are unacceptable to use in this situation.
INCORRECT: I took English this semester, however, I really wish I'd taken economics instead.
CORRECTED: I took English this semester; however, I really wish I'd taken economics instead.
Be aware that transitional expressions that don't separate independent clauses do not require semicolons.
CORRECT: You should know, however, that I'm not going to edit your paper for you.
Use a semicolon to avoid confusion in complicated lists.
Lists that contain punctuation within individual items should be separated with semicolons instead of commas to prevent reader confusion.
CORRECT: Present at the party were my mother, Jane; my sister, Erica; my boyfriend, Brian; and my roommate, Lauren.
Use a colon to introduce a list, an appositive8, or a direct quotation.
CORRECT: For the cake you will need the following: eggs, sugar, flour, and baking powder.
CORRECT: There is one food I refuse to eat: olives.
CORRECT: This was my grandmother's favorite piece of advice: "never go to bed angry."
Be aware that a colon needs to be preceded by a complete independent clause, not a sentence fragment.
Use a colon between independent clauses not joined by a coordinating conjunction.
This rule only applies if the two clauses are very closely related (e.g. the second one clarifies the first). If you're not sure that they're close enough to be joined by a colon, use a semicolon, comma, or period.
CORRECT: Getting hurt in relationships is inevitable: love is messy.
6 Some conjunctive adverbs: however, moreover, nevertheless, therefore, etc. (see Hacker for a complete list)
7 Some transitional phrases: after all, in other words, for example, etc. (see Hacker for a complete list)
8"Words or word groups that complete the meaning of the subject by either renaming it or describing it" (Hacker 487).