I've seen different questions related to the same verb, but those questions implicate an imperative form (For example when you tell somebody to log in/on) which I think may vary the condition in which the verb is used.

  1. He uses another program to log in to the system.

  2. He uses another program to log into the system.

  3. He uses another program to log on to the system.

  4. He uses another program to log onto the system.

I think number 2 is correct but I'll wait for your suggestions.

  • You may find some useful answers in this answered post or this one.
    – Nonnal
    Nov 30 '15 at 23:25
  • 1
    Use whatever preposition makes the metaphor work the best... Are you metaphorically “in” the system or “on” the system? I’m more likely to be in a chatroom than on a chatroom. But I’m more likely to be on the master console than in the master console.
    – Jim
    Nov 30 '15 at 23:29
  • 1
    I have read that topic and 'notaverb.com/login'. They say login is not a verb. We can't conjugate it. However what about 'log in'? He 'logs in' every day. He logged in yesterday. They say it still is not a verb but really I thought It came from 'log' (captain's log). You write in the log. So I thought the action itself was thought as log/v/ (logging). When I think about the action log in, I can totally relate into writing in a log (logging into a notebook, registering your name into somewhere). Comment section is not enough, got to many things I want to say and express myself better.
    – Grizzly
    Nov 30 '15 at 23:39
  • 1
    As a computer geek for 49 years, I prefer #1. Especially since "login" as a noun is widely accepted in the industry.
    – Hot Licks
    Dec 1 '15 at 7:43

For my money, log on to a system or log in to a system are interchangeable, and depend on the metaphor you are using (see comment on your post). I suppose there is a small bit of connotation that "log on" implies use, and "log in" implies access or a specific user.

Not to be confused with "login" - a noun describing a combination of username/password.

I'd pick 1) because the program is accessing a system (in vs. on), and "log in" is the action, whereas "to" begins the prepositional phrase.

So, he uses another program to log in to the system.

Not the answer you're looking for? Browse other questions tagged or ask your own question.