1) the "-해서", which is a contraction of "-하여서", is the part corresponding to "so". It gives a cause or reason for something - the class is boring to make him doze off. I think "The class is so boring it makes me doze off" or "The class is so boring it nearly puts me to sleep" might be a better translation of it. 2) 졸다 is the common verb for "doze off". 꾸벅꾸벅 is an adverb describing the way someone dozes, like the way the head bobs. It is one of the sound/action imitation words (onomatopoeia) which usually require -하다 or another verb to function as a verb (꾸벅꾸벅하다, 꾸벅꾸벅 졸다). Korean has a lot of these imitation words which describe things directly. ex: (가슴이) 철렁하다 (my heart sank), 화가 나 씩씩거리다 (fume with anger), 일을 덤벙덤벙 한다 (does his work haphazardly). 철렁, 씩씩, 덤벙덤벙 are all such adverbs directly imitating the way something happens. In this example "졸고 있어요" by itself will make sense too, but adding 꾸벅꾸벅 makes it really come alive. 3) As mentioned, -(아/어)서 (지루하여서) is a heavily used conjugation form which can mean: 1) something done immediately before another in sequence 2) reason/basis for certain action/event (as our example case), 3) a means or way of doing something. Ex: 1) 고기를 구워서 먹는다. (first broil the meat, then eat it); 짐을 덜어서 다른 차에 싣는다. 2) 강의가 지루해서 졸았다. 강이 깊어서 건널 수가 업다 (the river is too deep to cross). 3) 그는 걸어서 학교에 다닌다. (He goes to school on foot).