PASSIVE VOICE
We use passive voice to talk about an action where the subject isn't so interesting or the object of the sentence is the focus.
Normally, the subject of an active verb is not expressed in its corresponding passive sentence. If it is necessary, it is expressed using the word "by"; the noun that follows is called the 'agent'.
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It's the best option in situations in which the speaker wishes to draw attention to a fact or the receiver of said action instead of its doer.
In cases where the subject is unknown, it is a better option to use passive voice than to start a sentence with an expression to say that the identity is unknown. Using it to start sentences with familiar topics and then expressing unfamiliar information is also used. It's important to note that it is normal to go back and forth between the active and passive voice in a text.
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Examples:
A dog was elected mayor last year in our
The pyramids were built many years ago.
Her car had been fixed by then
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​​The structure for passive voice is:
Active voice (A)
Subject + verb + object
The teacher helped the student
Passive voice (P)
Subject (original object) + Verb to be + Verb in Past Participle + (by + agent (original subject)).
The student was helped by the teacher
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Examples using 'teacher' as subject, 'student' as object and 'help' as the main verb:
Simple forms are used to express actions that happened in the past, everyday actions, and actions that will happen somewhere in the future.
Examples:
Past simple
A: The teacher helped the student
P: The student was helped by the teacher
A: The teacher helps the student
P: The student is helped by the teacher
A: The teacher will help the student
P: The student will be helped by the teacher
Continuous/progressive forms (be + -ING) are used when we describe an event as 'in progress' or continuing (at a specific time or up to a specific time).
Past continuous
A: The teacher was helping the student
P: The student was being helped by the teacher
A: The teacher is helping the student
P: The student is helped by the teacher
Future continuous isn't usually used in passive form.
Perfect forms (have + past participle) are used to express a connection between two moments in time, either an earlier and a later past event, a past event and the present, two moments in the future, or that something is/was/will be completed by a particular time.
Past perfect
A: The teacher had helped the student
P: The student had been helped by the teacher
A: The teacher has helped the student
P: The student has been helped by the teacher
Future perfect isn't usually used in passive form.
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Modal verb
A: The teacher must help the student
P: The student must be helped by the teacher
Modal perfect
A: The teacher must have helped the student
P: The student must have been helped by the teacher
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References: