I am trying to become su in the terminal window. It asks for my password, I give my usual password and I get "authentication failure". I thought that my log in password works for the su as well? Am I wrong?

You are not correct. You are thinking of sudo, not su.

The su command requires the password of the account into which you are trying to switch. In the case of using su without specifying any account name the su utility will try to switch you to the root account so you would need to enter the password of the root account IF the root account has a password.

You appear to be using Ubuntu. In Ubuntu the default is that root does not have a password which makes using su to root impossible. That can be changed by giving a password to the root account.

For su the root password is needed. But you can tryThis way you are asked for your password.

that worked! thanks to all ...