Can't log in

[mikethebook]

The old gentleman next door has a MacBook Air running Sierra, I think. Somehow, I don't quite understand, he ended up with two login passwords but now neither of them will work. I booted into Recovery Mode to reset the password but after typing resetpassword in Terminal, the only window that came up was the failed login questions menu not the expected list of volumes with space for new passwords. I tried that a couple of times but it didn't work. So I tried to boot into Single-User mode but that didn't work no matter when I pressed Command-S. What worried me slightly was the speed with which the computer started and shut down but the gentleman told me that he had no apps or documents on the computer. He used it just for e-mails and browsing.
Can anyone suggest what might be wrong? Failing that, since he has nothing of value on the computer, how can I do a reinstall of Sierra or another OS without being able to login? I run a Mac myself.
Many thanks.

P.S. When I booted up pressing Alt, the only drive shown was the hard drive, no recovery drive as I had expected.

 

[benshive]

I remember that when I had to use the resetpassword command, there was an option in the window that appeared that said something like "Forgot all passwords" that let me reset the password for all user accounts without any security questions or passwords. If you find that you have to wipe it, if you boot into internet recovery (cmd + r on startup), you can use Disk Utility to wipe the internal drive and then install the version of macOS that came presinstalled on the device when it shipped. Then you can update to the latest version or whatever version you'd like.

 

[mikethebook]

Thanks. I think a clean installation is the way to go.