The wonders of Korean language. As you know in English verbs are supposedly action words and adjectives are words used to describe — in short descriptive words. In Korean, verb as I have always been mentioning is the most important part of the statement and is usually placed at the end.
Adjectives in Korean are formed by manipulating the form of a descriptive verb. An example is the word 예쁘 (yeppu which means pretty) the infinitive form of this descriptive verb is 예뻐 (yeppo). As discussed previously on creating the infinitive form of words, if it ends in ㅡ, the letter is dropped and ㅏ or ㅓ is added depending on the vowel before ㅡ, so in the case of yeppu its ㅖ by rule the infinitive form should end with ㅓ. So how is the adjective derived from this word? simply add ㄴ so its 예쁘+ㄴ = 예쁜 (yeppun or romanized as yeppeun in some cases).
The same thing is applied when transforming the word 기쁘 (gippu) to an adjective, it becomes 기쁜 (gippun or gippeun).
I have a feeling that there are other ways to transform words into adjective in Korean… maybe in the future lessons of the book I’d be able to find this out.