I was surprised to see so many people seated at the back of the class.
为什么用seated表被动 而不用sitting表主动.
答：这是推敲问题.
用sitting在这里也通的.
I was surprised to see so many people seated at the back of the class.
译:看见这么多人在班级的后排就座,我感到很惊讶.
析：be seated 表示状态.
I was surprised to see so many people sitting at the back of the class.
译:看见这么多人正坐在班级的后排,我感到很惊讶.
sitting 为现在分词,表示正在进行的动作. see sb. doing sth. 看见某人在做某事.