Question

When do you use a colon in a sentence?

Answer 1

1. Use colons after complete sentences
2. Use colons to format lists

Explanation:

https://www.quickanddirtytips.com/education/grammar/colons

Answer 2

To introduce a list or something important.

Explanation:

A colon's purpose is to introduce a list or a noun/action of interest. A good rule of thumb I've developed is that a colon can be replaced with a phrase like "and that is, [the thing]>" or "and those are [the list]".

Examples:

"He only had one chore left to do: feed the dog."

"He only had one chore left to do, and that was (to) feed the dog."

or

"There are 10 types of people in the world: those who know binary, and those who don't."

"There are 10 types of people in the world, and they are those who know binary, and those who don't."