The colon is needed to complete an unfinished statement or thought. The colon may be preceded by an independent clause (independent clause goes before the colon) and used in several situations. Notice that in certain situations, what follows the colon begins with a capital letter.
1) With a list of items
She checked her luggage again to make sure she had all the items she needed for the conference: two suits, two blouses, and two pairs of shoes.
BUT, if what precedes the list of items cannot stand alone, meaning that it is not an independent clause, do not use a colon.
Her luggage included two suits, two blouses, and two pairs of shoes.
2) Linkage of an independent clause with a independent, dependent clause, or phrase which may have one of several relationships: introduction to main idea, cause to effect, general statement to a specific example (only capitalize the first letter after the colon if what follows is a series of two or more sentences)
There is only one thing left for her to do: remembering where she placed the airplane ticket!
Before leaving her house, Sarah did not forget to do three things: Check that the stove is turned off. Turn off all electronics. Close all windows.
3) To introduce a quote (notice the capitalization).
Not only is Shakespeare a famous playwright of romantic plays, he also coined a famous saying about love that is still popular today: “Love is blind.”
A colon may also be used in certain forms of writing.
1) In an announcement (notice the capitalization)
Attention: Never leave your luggage unattended
2) In a business letter
Dear Mr. Smith:
3) In a play, skit, or court testimony
Ted: I have a phone conference with Mr. Cameron this afternoon.
Sarah: Oh, really?! I thought it was cancelled.
Ted: No, it will still take place.
4) In the title of a book or movie to set off the main title from the subtitle
X-Men Origins: Wolverine
The Fast and the Furious: Tokyo Drift
5) In time
9:30 pm or 21:30