root password half-working...
 

i cannot log in as root, as it won't accept the password. however, if i log in as another user, and i need to access a file that only root has access to, it will accept the password.
what do i do? i've never seen this before... and everyone i've asked has never seen this before either...

Are you on the machine or trying to connect remotely? If remotely, root is denied login from ssh by default, I think. If you are actually on the machine, when you login as regular user, what happens if you and give root password? If you have lost the root password, you can boot into single user mode and change root password.

See this article for another way...

Here is another thread about recovering root password.

i'm on the machine.
and typing su, then the password, does let into root.

and as for booting into single user and changing the root password, i've already tried that.

As root, add your user to the "wheel" group. You should have "su" then.

Where are you logging in at, linux console or graphical login? Could it be that your GDM/KDM configuration does not allow root to log in?
For GDM, as root, run gdmconfig, click on Security, check Allow Root to login with GDM.
For KDM, run kcontrol, click on System Administration, Login Manager, go to the Users tab and uncheck root.