"I heard that on the radio yesterday."

Translation: Mi aŭdis tion hieraŭ en la radio.

[AlexVaughnMiller]

I would have thought "per radio" the best way to put it since that is the medium being used.

[lyubomirv]

Why en ? Was I inside the radio? Would de or el be better?

[JamesTWils]

Why must it be "tion" instead of "tiun"?

[mihxal]

"Tion" corresponds to "that". "Tiu" would mean "that one" (often person).

[JamesTWils]

OK, then I suppose my question would have to do more with the English language. What is the distinction between that and that one?

[ponnuki]

"that one" or "tiun" would be either "that person" or "that thing of the class we are discussing". "That" or "tion" would be "that thing" in general. Perhaps it is easier to understand from questions: "That one" would be the answer to "who" or "which xxx", "that" would be the answer to "what".

[TreyEHarris]

Seems to be a lot of questions and no answers here as to why en la radio and not per la radio, sur la radio, de la radio — or I’d add even radie, which I see used with elsendi and the like frequently, but maybe is not used with aŭdi. (From other European languages, ĉe, tra and super seem like outside possibilities, too.) If it’s en, great, but is that just a standard but arbitrary convention? If not, is there a “logical” part of en’s definition that makes it the right choice here, or are these other choices real possibilities?

[KingLaSerna]

I would have said ''sur''.