1. 当不知道或没有必要指出动作的执行者时，常用被动语态，这时往往不用by短语。

The front window in the classroom was broken yesterday. 昨天，教室的前窗被打破了。(不知谁打破的)

They have been poorly paid. 他们的工资太低。(没必要指出工资是谁付的)

2. 突出或强调动作的承受者，如果需要说出动作的执行者，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. 这些书是专门为孩子们写的。(强调的是“”这些书)

3. 为了使语言得体或圆滑等不愿意说出动作的执行者。如：

You are requested to make a speech at next meeting. 请您在下次会议上作个发言。

It is said that she is going to be married to a foreigner. 据说她要嫁给一个外国人。

4. 出于修辞的原因，或是说为了更好地安排句子。如：

It is generally considered impolite to ask one’s age, salary, marriage, etc. . 问别人的年龄、工资、婚姻状况等通常被认为是不礼貌的。

The notion is especially opposed to by those who have invested much in the area. 这个提议特别遭到了那些在本地区投资很大的人的反对。(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显得句子松散。)

（来源：网络）