“During the break between sections 1 and 2, if you need to use the restroom, you may do so.”

You are taking a TOEFL test. The person who is administering the test is reading the test instructions aloud. She reads the following line about the bathroom break policy.

During the break between sections 1 and 2, if you need to use the restroom, you may do so.

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during (an event or time period), (independent clause)

This phrase is used when you want to express that an event happens within a certain period of time. For example:

During the month of May, we'll be introducing several new improvements to the site.

You can also use the order "(independent clause) during (an event)"

It's really annoying when people talk during a movie.

a break

"A break" is a short period of rest between periods of work.

use the restroom

In casual speech, people say "go to the bathroom" or "use the toilet". "Use the restroom" is a polite formal way of describing this action in American English

do so

This is a formal way of saying "do that". Before using "do so", you need to introduce an action:

If you're interested in buying stocks, you can do so with the help of a broker

In casual speech, "do so" is replaced by "do that" or "do it"