You just installed Linux Mint – everything is hunky dory. All of a sudden you can’t log back in! You have no clue, you even have a note with a piece paper for your password. A common mistake is that you may have switched keyboards, in turn, the password from the install may have switched locations. For instance, if you switch between languages (German/French etc) and even nuances of languages (British English/American English) you might have double check your keyboard output because if you used special characters or symbols, these might have moved around on the keyboard. Here’s a good post to check to see you through this:

Linux Mint won’t install on my password or Linux Mint password doesn’t work anymore

The Linux password system does not change itself, you need to be superuser, so whatever password you had is still there, it’s just not going in properly. Test your knowledge. Attempt a console login with the Ctrl+Alt+F4 keypress and prove your user name and password. Switch back to a graphic login with the Ctrl+Alt+F8 keypress. The greeter screen can be confusing, leading you to input your password as a user name, or your user name as a password. Read the prompt and input what it is asking for. Hit CapsLock a few times until the light is on, then once more to switch it off. It is possible the BIOS and Linux disagree over the keyboard state. In general, stick to ASCII letters and numbers in your password, some symbols like @ and “ or £ and # move around on the keyboard. You type @ but Linux sees “ for example. Changing your password behind-the-scenes using this method https://sites.google.com/site/easylinuxtipsproject/5 can lead to more problems if your home folder is encrypted.