In some sentences the subject “you” is simply understood:
Close the door. = You close the door .
Be quiet! = You be quiet!
Take out the garbage. = You take out the garbage.
To show that you understand this, your teacher may want you to always write in, and underline once, the understood “you.”
Note that "I" or "We" is also sometimes understood:
Thank you! = I thank you! or We thank you!