Unless I'm mistaken, she was back at work yesterday

Unless I'm mistaken, she was back at work yesterday

 Is the sentence right? Why back at work, not to work?

Both are possible.

'at work' - You are thinking of 'work' as a place, eg an office downtown somewhere.

'to work' - you are thinking of work as a task (eg write a report, sweep a floor, drive a bus)

If we are talking about Mary who stayed home sick yesterday, we normally say 'back at work'.

Best wishes, Clive

So if Mary stays home today, we would say: Tomorrow, Mary will back to work, wouldn't we?

No. If you are thinking that Mary's location tomorrow will be different than it is today, say 'Mary will be back at work'.

You could also say 'She will go back to work', in the same way as you mught say 'Next year I will go to China'.

In other words, you go to a place. After you arrive, you are at a place.

Best wishes, Clive