Debian: single mode & reset password root

• First screen - grub - press e
• Modify kernel line: add single between ro quiet and at the end of this line add init=/bin/bash
• Press F10
• When the prompt is root@(none):/# you have to remount the / partition to have read-write access:

mount / -rw -o remount

• Now you are ready to modify the root password: type passwd and change it!