[نهــال]

Using the passive

Usually the passive is used without a by-phrase. The passive is most frequently used when it is not or not important to know exactly who perform an action

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A) Rice is grown in India

No need to write rice is grown in India by people ,by famers,by someone… because It is not important to know exactly who grows rice in India

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B)Life on the Missisippi was written by Mark Twain

The by-phrase is included only if it is important to know who performs an action,so the phrase ( by Mark Twain) in ex.B includes important information

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c)This biscuit was made by my daughter

Sometimes, even when the speaker knows who performs an action, s/he chooses the passive with the by-phrase because s/he wants to focus attention on the subject of the sentence.
In ex.C the focus of a attention is on the biscuit

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The structure of the passive is

Object+ helping verb+ past participle+by+subject