1. 當(dāng)不知道或沒有必要指出動(dòng)作的執(zhí)行者時(shí)，常用被動(dòng)語態(tài)，這時(shí)往往不用by短語。

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

They have been poorly paid. 他們的工資太低。(沒必要指出工資是誰付的)

2. 突出或強(qiáng)調(diào)動(dòng)作的承受者，如果需要說出動(dòng)作的執(zhí)行者，用by短語。

The time-table has been changed. 時(shí)間表已變動(dòng)了。(要突出的是“時(shí)間”)

These books are written especially for children. 這些書是專門為孩子們寫的。(強(qiáng)調(diào)的是“”這些書)

3. 為了使語言得體或圓滑等不愿意說出動(dòng)作的執(zhí)行者。如：

You are requested to make a speech at next meeting. 請(qǐng)您在下次會(huì)議上作個(gè)發(fā)言。

It is said that she is going to be married to a foreigner. 據(jù)說她要嫁給一個(gè)外國人。

4. 出于修辭的原因，或是說為了更好地安排句子。如：

It is generally considered impolite to ask one’s age, salary, marriage, etc. . 問別人的年齡、工資、婚姻狀況等通常被認(rèn)為是不禮貌的。

The notion is especially opposed to by those who have invested much in the area. 這個(gè)提議特別遭到了那些在本地區(qū)投資很大的人的反對(duì)。(因those的定語太長了，若用它作主語，主語與謂語就相距太遠(yuǎn)而顯得句子松散。)

（來源：網(wǎng)絡(luò)）