see sb doing sth 改成被动

sb is/was seen doing sth

或者sb was/is seen to do sth

前者表动作，后者表状态

Sb.is seen doing sth某人被看到正在做某事.

楼主那道“比如”答案应该是
Mary is seen playing the voilin.

sb be seen to do sth
sb be seen doing sth

两者的区别是后者表示动作正在进行。