lol May I ask where you are originally from? Your knowledge of Chinese letters seem to put you on a super fast-track for sure! Good for you!

As per the lesson notes: "For ㄷ special case verbs, those where the final ㄷ becomes a ㄹ, add 을 to the end of the stem. 듣다=>들을"

They haven't worried about the past, they don't worry about the present, it will be the future that makes them tense.