Well, here’s how you use these two pieces of punctuation:
A semicolon is used to join two complete sentences which are very closely related in meaning. Example: Mark my words; you’ll learn a lot in her language arts class! In this compound sentence, the two pieces need each other because without the explanation, “you’ll learn a lot in her language arts class,” the statement “mark my words” wouldn’t make any sense. The two connected sentences are depending on one another.
A colon is used after a salutation in a block or semi-block letter. It can also be used between numbers, to indicate the time of day. Sometimes it is used to introduce a list. Example: The following authors have eBooks available for purchase: Rick Riordan, Nicholas Sparks, Suzanne Collins, Becca Fitzpatrick, and Lauren Kate . Another example: Dear Webmaster: Or Dear Customer Service Manager: And don’t forget this: It’s 9:12 a.m.
I tell my students to take a good look at what a colon looks like. The prefix semi- means only partly. So a semicolon only has the top half of the colon. That seems to help my students identify the two pieces of punctuation by sight. But it’s still a long stretch to get seventh graders to understand how to use semicolons in a sentence they might create themselves.