Root

Login as root

Can be used with root locked

sudo --login

Short version

sudo -i

For users without root privileges, but we know the root password (can't be used if root is locked):

su -

su -l

su --login 

Add root password

If root doesn't have a password, we can use:

sudo passwd root

Lock root

sudo passwd --lock root

Short version

sudo passwd -l root

Unlock root

sudo passwd --unlock root

Short version

sudo passwd -u root