In language arts we had to do a project on grammar, and I chose when to use a colon. I had to work on a Keynote presentation. Some of the main rules that you need to know for when to use a colon is: only use a colon after a complete sentence. When you use a colon do not put a coordinating conjunction, like the words: and, but or for. Colons introduce lists and other nouns. One of the colon’s main uses is separating the author from the introduction.