Booting into single user mode by default
Solution 1:
Open Terminal.app and enter:
sudo nvram boot-args="-s"
In the future your Mac will always boot to single user mode until you remove (sudo nvram -d boot-args or sudo nvram boot-args="") or replace the boot-args.
Other boot-args directly modifying the boot mode:
sudo nvram boot-args="-v": verbosesudo nvram boot-args="-x": safe mode