Which preposition should I use when talking about some work using a computer?
I worked on my computer yesterday (that's what I heard a native speaker say)
However, my Russian book gives the following example:
When you work at a computer, you watch its operations on the monitor".
Are both possible?
Thank you in advance.
Both are possible.
Working "at" has more to do with your environment. If you work at a computer all day - references to posture, eye strain.
Also, I do agree that if you don't actually DO much, but you are monitoring the activity, that is working "at."
If you are working on your computer, you could either be doing things to the computer itself (to make it function better) or because that's what you use to get most of your work done. That certainly applies to me. I work on a computer -- if we have a power outage, I'm pretty useless, since it has most of my files.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.