passive voice 是不是不一定要加by或with的

Passive voice 不一定要加by或with. 許多 passives have no agent. (doer)

Passives frequently occur without an agent phrase and are called agentless passives. The entity responsible for an action may not be known or may not be relevant, or may simply too obvious.

(1) 當做者( agent) 很明顯
The streets are swept every day. (清道夫)
She was fined $300 for speeding. (法官)

(2) 當不知道誰人做
My car was stolen.
You will be met at the station.

(3) 當主語 (subject) 是許多人 (people)
Many people were killed in the Second World War.
158 passengers were killed in the plane crash.

(4) 當作者不想提及誰人做, 因為某種原因
The most popular supervisor was asked to leave the company.
It was decided that it would be too costly to take the case to trial.
The letter has been opened! (可能是講者開了封信)

(5) 當 give instructions, warning, 在考試, 填表格, 使用手冊
All the questions must be answered.
The wine must be kept cool.
Swimming in the lake is prohibited.
Application form should be received not later than Friday, November 30, 2012

(6) 當我們興趣在主語 (subject) 上, 在 action上, 誰人做不重要
Eton College was founded in 1440.
A new library is being built in our town.
The President was called upon to make a speech. (誰人 called President不重要)
New features are being added to our machine.

(7) 有動詞在某種用法上只用 passive voice, 如果加上 agent, 句子變得怪異的, 這類 passive 是沒有 agent.
Paper is made from wood.
She was born in Hong Kong.
(born 在這句不是 adjective ~ used only in passive, without by)

不是，要看情況

例如這樣的時候，當subject並不重要的時候，就不需要
example: He was arrested for murder.
（一定是警察拘捕他的）

可是當subject也有它的作用的時候，就需要
example: He was killed by Tom.
(Tom 是謀殺犯，所以是重要的資訊)

不一定,看情怳先睇下洗吾洗加