The sentence is correct. It just means that 듣다 conjugates irregularly. Some verbs (especially those with 받침, or "bottom consonant", in the last syllable of its stem) has this characteristic. 듣다 is a case called ㄷ irregular conjugation (ㄷ 불규칙 활용) as it's related to the ㄷ 받침. The ㄷ 받침 changes to ㄹ if a conjugational suffix starting with a vowel follows it. - 듣다 + 어XX => 들어XX (들어, 들었어, 들었지만). - 듣다 + 으XX => 들으XX (들은, 들을, 들으면). - 듣고, 듣지, 듣는, 듣더라도 - no irregularity if no vowel. There are many different irregular verbs, which are explained in most grammar books and sites. Here's one: http://www.howtostudykorean.com/unit1/unit-1-lessons-1-8/unit-1-lesson-7/ By the way, a better sentence for this example would be: 작은 소리를 못 들어요 (I can't hear low sounds).