Боб Уайтман

How could you miss this awful sentence?

When I was sick, he read my paper for (instead of) me at the congress.

The site claims that the correct Russian would be:
Когда я заболел, он выступил за мной
на съезде.

Which is complete nonsense.

За мной there implies either the physical location like behind me or a sequence like right after me

But the whole sentence conveys the fact that the man took the sick man's place and did the sick man's job. So it has to be:

Когда я заболел, он выступил за меня
на съезде.