Learn Croatian: Slušali smo vijesti na radiju. - We listened to the news on the radio.

In English, "news" is an uncountable noun treated as a singular. In Croatian, vijest is a regular feminine noun meaning "a piece of news". In this sentence, vijesti is the accusative plural form, because you are literally listening to "pieces of news".

The word for radio is "radio", so why does it change to "radiju" instead of "radiu"?

Even though radio ends in -o, it is a borrowed word and acts as a masculine noun, not neuter. When placed in the locative case after the preposition na, it takes the ending -u. However, Croatian generally avoids having the vowels 'i' and 'u' right next to each other in this noun declension, so a -j- is inserted to make it radiju.

Could I use "čuli smo" instead of "slušali smo" in this sentence?

You could, but it changes the meaning. Slušali smo comes from the imperfective verb slušati (to listen), which implies the ongoing activity of paying attention to the broadcast. If you used čuli smo (from the perfective verb čuti, meaning to hear), it would mean "We heard the news on the radio", focusing on the single event of receiving the information rather than the activity of listening.

Why do we use the preposition "na" for the radio?