The '르' Irregular Verbs

When a verb stem ending in 르 meets an ending starting with 아/어 (like -아요 or -어요), it undergoes a special change.

The ㅡ vowel in 르 drops completely.

An extra ㄹ is added to the bottom of the previous syllable block.

If the vowel before 르 is 아 or 오 , the ending becomes 라 . For other vowels, it becomes 러 .

Before consonant endings like -고 or -(으)면, no change happens.

Core takeaway: 르 changes to 라 or 러

모르다 is one of the most common 르 irregular verbs.

Before endings starting with a consonant like -고 or -(으)면, it stays regular:

모르다 → 모르고

모르다 → 모르면

However, before -아요 or -았어요, the ㅡ drops, and an extra ㄹ is added to the first syllable (모), making it 몰 . Since '모' has an '오' vowel, it takes 라 .

Base Form · Present (-아요) · Past (-았어요)

모르다 · 몰라요 · 몰랐어요

Let's look at other high-frequency verbs that follow this exact same rule.

부르다 (to call/sing) changes to 불러요 and 불렀어요 because the vowel before it is '우', taking the '러' form.

고르다 (to choose) changes to 골라요 and 골랐어요 because the vowel before it is '오', taking the '라' form.

Base Form · + 고 · + 아요/어요

부르다 · 부르고 · 불러요

고르다 · 고르고 · 골라요

빠르다 · 빠르고 · 빨라요

A common beginner mistake is simply dropping ㅡ without adding the extra ㄹ , or just attaching 어요 directly without changing the stem at all.

Incorrect: 모라요 / 모르어요

Correct: 몰라요

Incorrect: 부러요 / 부르어요

Correct: 불러요

Always remember that the 르 irregular requires that extra ㄹ on the previous block!