*굽다 means "be curved, bend, be stooped". It is intransitive verb. It is used with a subject.(no object) e.g. 그는 늙어서 허리가 굽었다. He is bent with age./He stoops from age. 그는 선천적 기형으로 등이 굽었다 He has a bent back as a birth defect . 굽히다 and 구부리다 are causative verbs of 굽다. Both are transitive verbs used with an object. *굽히다 has two usages 1. bend(one's back), bow(one's head)(human body or a part of body) e.g. 허리를 굽혀 손을 발가락에 대 보시오 Bend over and touch your toes. 그는 신발 끈을 묶기 위해 몸을 굽혔다 He stooped down to tie his shoes. 2. deviate[deflect] from(one's principle), yield, submit e.g. 그는 끝까지 자신의 뜻을 굽히지 않았다 He stood firm in his views. 테러리스트들은 자신들의 요구를 굽히지 않고 있다 The terrorists are unyielding in their demands. *구부리다 means "bend, curve, crook, twist"(animal,human body or material) e.g. 그들은 숟가락, 나이프, 포크를 구부렸다. They bent spoons, knives , and forks. 고양이가 등을 활처럼 구부렸다. The cat arched its back 굽히다 is commonly used when the subject has the intention of bending and the object is a part of his own body or his own spirit/soul 구부리다 is commonly used when the object is not a part of the human subject or the subject has no intention of bending.