Usually „na” is with activities, events (like mass, lesson, lecture, conference) or the places that are considered to be rather flat surface like a parking. „W” is used with buildings like church, school, conference center. If you use „na” with them it meant you are on their roofs.
Yeah, it seems so. Removed 'on', added 'in' instead as apparently this is correct.
"In" is only correct if it's a small conference like a meeting, not a big conference like an international gathering. Does Polish distinguish between the two? If not, I'd say that both "in" and "on" (and "at") are acceptable in English, but with subtly different implications for the nature of the conference.
"On a conference" is fine, actually. Much like we can say "I am on holiday", we can also say "I'm on a conference". This is a conference as in a large meeting, e.g. of scientists in a big hall, rather than a small meeting of people in private e.g. in a board room. The small conference would require "in", but "on" is perfectly fine for the big conference. Source: I'm an Anglophone who goes on a lot of conferences!
As a fellow Anglophone, might I enquire which variant of English is the one which you speak? I go to a lot of conferences too, mainly in Europe, and have never heard this usage; also, I am unaware of the distinction between large and small conferences taking a different preposition.
Of course, I am aware of being on a conference call! I've done way too many of those!