Seat的用法I saw him seated at the back of the classroom.I saw h

seat的用法比较特殊．当动词时有两种用法：
1．be seated.例：they are seated there.他们坐在那．
2．seat oneself.例：they seat themselves there.他们坐在那．
这也就可以解释你写的这两句了,第一句是用法一,see sb.done.省略be seated 中的be．第二句则是用法二,see sb.doing．
同样用法的词还有：hide（隐藏） 和 station(驻扎)