Learn Swedish: Vi lyssnar inte på radion. - We are not listening to the radio.

In a simple Swedish sentence, the verb is almost always the second piece of information. The negative word inte (not) comes right after that verb. So we say Vi lyssnar inte instead of translating "We not listen" word-for-word.

Why doesn't the Swedish sentence have a word for "are" like in "are not listening"?

Swedish does not have an "are doing" form. The single present tense verb lyssnar means both "listen" and "are listening". You should never use the Swedish word for "are" (är) together with another verb like this!

Is på always required here?

Yes! Just like in English where you "listen to" something, in Swedish you lyssnar på something. Without på, the sentence would just mean "We are not listening the radio."

How do we know radion means "the radio" and not just "radio"?

In Swedish, you don't usually put a separate word for "the" in front of a noun. Instead, you add an ending. The base word is radio, and adding -n at the end makes it radion ("the radio").