“Take a moment to answer those questions on page 13 if you haven't done so already.”
You're a teacher. You're teaching your class with a textbook. Some of the students are moving quickly through the textbook, while some are moving more slowly. You want to tell the slower students to answer some questions. You say this.
Take a moment to answer those questions on page 13 if you haven't done so already.
Use this phrase when you want someone to do something for a short time, like one or two minutes:
Take a moment to look over the sheet I just passed out.
Take a moment to think back to the last time someone paid you a compliment. How did it feel?
A "moment" is a short length of time which can't be specifically measured. It's also used in other phrases like:
You "answer questions" in a textbook, on a test, on a homework assignment, etc.
This phrase refers to an action that you've just talked about in the last 1-2 sentences. It means "do that".
You must pay within 15 days. If you do not do so, you will be charged a 10% fine.
Send it to me immediately if you have not already done so.
"Do so" sounds formal.