"그는 가운데에 앉아요."
Translation:He sits in the center.
In this respect, sit isn't an action verb. This version if sit refers to the state of being sat down. Since the action of sitting (physically lowering the body to a sitting position) isnt generally what the speaker would mean, it cant be 에서 since it's not an action verb
It really depends on where you need them, but in general they will go at the end of the sentence. Sentence structures are Subject/Object/Verb in Korean. Here's an ex. with 한국어(korean) and 공부하다(to study)
O 한국어를 공부 하는 중이에요 = I'm studying Korean now/I'm in the middle of studying Korean (this is right)
X 나는 공부해 한국어를 다른 사람과 같이 != I study Korean with some other people (this is wrong)