Reset admin password without the old password
You can reset the password in Mojave by entering recovery mode and using the "resetpassword" utility via the terminal:


Boot your Mac while holding CommandR. When you get the Apple logo release the keys and allow the Mac to come up in Recovery Mode.
When you get to the macOS Utilities dialog select Terminal from the Utilities menu
Type resetpassword into the terminal window. In a few seconds the reset password utility will open and allow you to reset the password on your account.