-어하다 is used to make an adjective into a verb. 외롭다(be lonely) => 외로워하다(feel lonely) 슬프다(be sad) => 슬퍼하다(feed sad) 좋다(be good) => 좋아하다(like) 아프다(be sick)=>아파하다(feel sick) c.f. -어지다 is also used to make an adjective into a verb. 외롭다(be lonely) => 외로워지다(become lonely) 예쁘다(be pretty) => 예뻐지다(become pretty) 슬프다(be sad) => 슬퍼지다(become sad)

When someone is saying their own thoughts they use words on the left side. For example, 가을이라서 그런지 기분이 우울해(우울하-+-여) I feel blue when autumn comes. (Note that it's not correct to say 가을이 오면 난 기분이 우울해해(우울해하+-여) ) 나 요새 너무 외로워. I'm so lonely these days 네가 가면 난 너무 슬퍼. I'm so sad when you're gone. 2NE1 콘서트 간다고? 부럽다! Are you going to 2NE1's concert? How lucky you are!