Linux: 'su' VS 'sudo' VS 'sudo -i'

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su

enter root mode using root’s password

sudo

use super user’s privilege to execuate a command

sudo -i

enter root mode using a superuser’s password

su VS sudo -i

The passwords used are different
su: must use root’s password
sudo -i: can use any user’s password who is in admin group

Enable/Disable root user

Some systems did not enable root user (Some users in admin group).
Give root password will enable root

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$sudo –i passwd root
Enter new UNIX password: [Set new password for root account]
Retype new UNIX password: [Confirm new password]

Delete password of root will disable root

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$sudo passwd -dl root