What do the letters of the su command stand for? (Hint: It is not Superuser.) What can you do using su besides give yourself root privileges? How would you log in as Zach if you did not know his password but knew the root pass- word? How would you establish the same environment that Zach has when he first logs in?

What will be an ideal response?

The letters stand for substitute user. You can give yourself privileges of any
user whose password you know (or any user when you are running with
root privileges). You can also execute a command (other than a shell) as the
specified user.
To log in as Zach, you would first log in as root, and then give the command
su zach or su – zach to establish the same environment that Zach has when
he logs in.