나쁘+아?
"If the last vowel of the stem is not ㅏ (ㅑ) or ㅗ (ㅛ): +어"

But in the adjective 나쁘 the last vowel of the stem is "eu" so i should add +어 at the end. However in the book put that i sould add +아. Why? :S

Question

나쁘+아?
"If the last vowel of the stem is not ㅏ (ㅑ) or ㅗ (ㅛ): +어"

But in  the adjective 나쁘 the last vowel of the stem is "eu" so i should  add +어 at the end. However in the book put that i sould add +아. Why? :S

Answer

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

Answer

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.