to perform the actions that allow you to begin using a computer system 登入; to allow sb to begin using a computer system
log in / on
to perform the actions that allow you to begin using a computer system 登入
You need a password to log on.
log sb in / on
to allow sb to begin using a computer system
The system is unable to log you on.
Definitions from the Oxford Advanced Learner's Dictionary, 9th edition