1)我们不知道或没有必要说明动作的执行者是谁的时候(这时不带by引起的短语)。

　　Printing was introducedsintosEurope from China. Look! There is nothing here. Everything has been taken away.

　　2)当我们强调或兴趣在动作的承受者时(这时可带by引起的短语)。

　　The song was composed by a student. A good time was had by all.

　　3)当我们出于礼貌、措辞圆通等方面的考虑不愿说出动作的执行者是谁时。

　　You are requested to give a performance. You are cordially invited to a party to be given at the Teachers' Club at 3 p.m. Nov. 23.

　　4)当出于行文的需要，被动结构更容易安排句子时。

　　They are going to build an apartment house here next year. It is going to be built right beside the Office Building. The plan was especially supported by those who wished to have more chance to speak English.