When the final vowel of a verb stem is 'ㅡ', 'ㅡ' is omitted before '-어/-아'. If the verb stem is one syllable, '-어' is added as the rule. If the verb stem is more than one syllable, the rule applies to the vowel of previous syllable. 고프다=>고파 나쁘다=>나빠 바쁘다=>바빠 담그다=>담가 따르다=>따라 크다=>커 뜨다=>떠 끄다=>꺼 슬프다=>슬퍼 기쁘다=>기뻐 예쁘다=>예뻐 미쁘다=>미뻐

If the final vowel in a stem is ㅡ then you have to look at the previous syllable to find what to add. With 나쁘다, you look at 나 and therefore add 아. 나빠요. With 예쁘다, you look at 예 and therefore add 어. 예뻐요.