In some verbs the stem final ㄷ (before the plain 다 verbal ending) changes to ㄹ before a vowel ( ㄷ irregular) and in some other verbs it does not (ㄷ regular). Here are a few of each, according to a source I just looked at.
ㄷ Irregular (ㄷ > ㄹbefore vowel) 듣다 to listen 걷다 to walk 묻다 to ask 싣다 to load 깨닫다 to perceive, realize
ㄷ Regular (ㄷ > no change) 닫다 to close 받다 to receive 믿다 to believe
Yes you are right! Duolingo always throws us into something we don't know yet without giving any explanation beforehand.
This is called ㄷ irregular rule.
I suggest you study irregulars firstly somewhere else before proceeding with Duolingo, like I did, otherwise you'll get lost.