当不了解或没必要指出动作的实行者时，常用被动语态，这个时候总是不需要by短语。
The front window in the classroom was broken yesterday. 昨天，教室的前窗被打破了。(不知哪个打破的)
They have been poorly paid. 他们的薪资太低。(不必指出薪资是哪个付的)
突出或强调动作的承受者，假如需要说出动作的实行者，用by短语。
The time|table has been changed. 时间表已变动了。(要突出的是时间)
These books are written especially for children. 这类书是专门为孩子们写的。(强调的是这类书)
为了使语言得体或圆滑等不想说出动作的实行者。如：
You are requested to make a speech at next meeting. 请你在下次会议上作个发言。
It is said that she is going to be married to a foreigner. 据了解她要嫁给一个外国人。
出于修辞是什么原因，或是说为了更好地安排句子。如：
It is generally considered impolite to ask ones age, salary, marriage, etc. . 问其他人的年龄、薪资、婚姻情况等一般被觉得是不礼貌的。
The notion is especially opposed to by those who have invested much in the area. 这个建议特别遭到了那些在当地区投资非常大的人的反对。(因those的定语太长了，若用它作主语，主语与谓语就相距太远而看上去句子松散。)
