（1）当不知道或没有必要指出动作的执行者时，这时往往不用by 短语。
如：
Mr. White, the cup was broken after class.
怀特先生，这只茶杯是下课后被打碎的。
（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。
如：
These records were made by John Denver.
这些报道是被约翰丹佛做的。
被动语态中谓语动词的基本构成
be + done
被动语态的构成
（1）一般现在时：am / is / are + done。
如：
①People grow rice in the south of the country.
Rice is grown in the south of the country.
稻米在这个国家的南部被种植。
②The school doesnt allow us to enter the chemistry lab without a teacher.
We are not allowed to enter the chemistry lab without a teacher.
我们不被允许在没有老师的情况下进入化学实验室。
（2）一般过去时：was / were + done。
如：
①They agreed on the building of a new car factory last month.
The building of a new car factory was agreed on last month.
建立一个新的汽车工厂在上个月被一致同意。
②The students didnt forget their lessons easily.
Their lessons were not easily forgotten.
他们的功课没有被轻易遗忘。
（3）一般将来时：will + be done。
如：
①They will send cars abroad by sea.
Cars will be sent abroad by sea.
汽车通过海路被送到国外。
②She will take care of theses children.
These children will be taken care of by her.
那些孩子将由她照顾。
（4）现在完成时：have / has + been + done。如：
①Someone has told me the sports meeting might be put off.
I have been told the sports meeting might be put off.
我已经被告诉会议可能会被延迟。
②He has brought his book here.
His book has been brought here.
他的书被带到了这里。
（5）含有情态动词的被动语态：情态动词+ be+ done。
如：
①You must hand in your compositions after class.
Your compositions must be handed in after class.
你的作文必须在下课之后被上交。
②He can write a great many letters with the computer.
A great many letters can be written with the computer by him.
很多信能够被他用计算机写。