The 'ㅡ' Drop Rule
In the regular pattern taught here, when a verb or adjective stem ends in ㅡ and combines with -아요 or -어요 , the final ㅡ drops.
After the ㅡ drops, the preceding vowel helps determine whether the form follows the -아 or -어 pattern. If there is no preceding vowel, -어 is the default.
Single-Syllable Stems
If the stem has only one syllable, like 쓰 (from 쓰다) or 크 (from 크다), dropping the ㅡ leaves no other vowel behind to help you choose.
When there is no preceding vowel to look at, the default choice is -어요 .
Infinitive · Stem + Ending · Conjugated Form
쓰다 · 쓰 + 어요 · 써요
크다 · 크 + 어요 · 커요
Multi-Syllable Stems
If the stem has more than one syllable, check the vowel in the syllable immediately before the final ㅡ .
If that preceding vowel is ㅏ or ㅗ , follow the -아 pattern.
바쁘다: preceding vowel ㅏ → 바빠요
아프다: preceding vowel ㅏ → 아파요
If the preceding vowel is not ㅏ or ㅗ , follow the -어 pattern.
예쁘다: preceding vowel ㅖ → 예뻐요
슬프다: preceding vowel ㅡ → 슬퍼요
Common Learner Mistake
A frequent mistake is keeping the ㅡ and simply attaching the ending next to it.
For example, writing 쓰어요 instead of 써요 , or 바쁘아요 instead of 바빠요 .
Remember that the ㅡ must completely disappear before the new ending attaches. This is a standard rule for these words, not a random exception.