Why does "radio" turn to "radia"? In one of the earlier exercises, "radia" was "radio".
Because the verb "słuchać" requires the genitive case, and the genitive of neuter nouns ending in -o (or in -e) is formed by changing the ending to -a.
I'm not sure that's a good tip. Thinking of it as being to the radio would suggest that we should use the dative.
'To' could be anything, like 'do' (preposition)... Anyway, as long as you remember it's not accusative