Separate the 2 problems. First solve the problem with user login, then you can work on the sudo problem.

From a root terminal try to enter
Of course,replace username with your user name. Does it work? If not, post the error it returns alone with the contents of /var/log/auth.log file

tl;dr
sorry.

but:
that sentence in itself is so wrong! example

all in all, i'm getting the distinct impression that your system is broken in several ways, and

1. a good linux tutorial for newbs
2. a reinstall

might be the best course of action.

Ondoho--I'm doing my best to solve a problem here. I'm appreciative of the help I have received. I'm not a techie but I'm doing my best. The problem is simply to get into my system using my non-root password. That's all I set out to solve. If I've not used the correct nomenclature in describing the situation, I'm sorry for that.

it's not about nomenclature, it's about "being root to get online". while the creation of a connection requires superuser privileges, you should never be root when you're using the internet. too dangerous.

Maybe you could post If your system really is broken it could be because there are repositories added that are not supposed to be there.

Here is reply from 'su username'

root@yama:~# su yuhan
yuhan@yama:/root$

If you run the su command again i.e.
yuhan@yama:/root$ su yuhan

You should be prompted for yuhan's password. If you enter the password is it accepted. Passwords entered at the prompt are not echoed so you are typing in the blind.

Sorry...that was a screw up!

You did not follow my instructions or posted the wrong information.
Were you prompted for a password?

Got this:

Password:
su: Authentication failure

Ok, what happens when you try to change the password.

So you could login as yuhan successfully. Then the issue is in the password itself it seems. From a root terminal try to set a simple password containing only digits using
Something like 123. Don't use numpad. Then try to login as yuhan again. Again, if you fail to login then post the contents of /var/log/auth.log

This is weird. I'm thinking about two possibilities: maybe your system is set up in such a way that each user (root and your regular user) uses a different locale, hence the keys you input are different for each user, even if the keys you're pressing on the keyboard are the same. After logging in on the CLI as your regular user (by executing su yuhan), try to write your user password as if it was any command (no need to press 'enter') to see if the keys you input match what is displayed on the screen.

The other possibility is, your user might have some privileges disabled, or there might be some permissions issue with some files.

Yes, weird...that's the word for it! I tried typing in my password as you suggested and, yes, the keys that I input matched what was on the screen. No problem there at all. As far as privileges disabled, I've no idea what can be done about that. What does the command ALL=(ALL)ALL mean? Does it have any bearing at all on this situation? On a post from another forum (Ubuntu) someone was describing a similar problem and a respondant suggested typing this in a terminal.

Well, this is embarrassing...and a little humbling. The solution to this problem was much simpler than I ever imagined. I figured it out entirely by accident! The problem was that I using an uppercase "Y" in my username and it was in fact a lowercase "y"! Once I had that right--voila! I had no problem. Terribly sorry for all the inconvenience. It's easy to forget that Linux is case sensitive. Thank you one and all for your help. Just sorry this proved to be such a dumb solution! My bad. Thanks again.