In passive voice, the subject of the sentence receives the action of the verb. It is usually made by using the verb to be or sometimes to get and the past participle of the main verb. The agent of the action does not always have to be expressed, but when it is, this is usually done using an expression with by.
If all this seems confusing, a few examples may explain it. (You can also watch this basic grammar video: http://www.youtube.com/watch?v=ePfmgMTgXl8)
Here is an active sentence:
Mars invaded Earth.
Here is that sentence in passive, in two forms:
Earth was invaded by Mars. Earth got invaded by Mars.
The past participle is the form of the verb that usually ends with -ed, although there are many irregular verbs. For example: Cook, cooked. Love, loved. Use, used. See, saw. Eat, eaten. Buy, bought.
(The present participle is the form of the verb that ends with -ing, and there are no exceptions: Cook, cooking. Love, loving. Use, using. See, seeing. Eat, eating. Buy, buying. This form is often used in continuous or progressive tenses, a tense that can be confused with passive voice. He was eating is active voice. He was being eaten is passive voice.)
Passive voice can be used with most tenses, but not all verbs can be used in passive voice. For example, Earth died after the invasion. This can’t be changed into Earth was died after the invasion. This is more a matter of meaning than grammar.
Here are active and passive voice sentences in various tenses, aspects, and moods:
Mars invades Earth. Earth is invaded by Mars. (Present Simple)
Mars is invading Earth. Earth is being invaded by Mars. (Present Continuous or Progressive)
Mars invaded Earth. Earth was invaded by Mars. (Past Simple)
Mars was invading Earth. Earth was being invaded by Mars. (Past Continuous or Progressive)
Mars will invade Earth. Earth will be invaded by Mars. (Future Simple)
Mars is going to invade Earth. Earth is going to be invaded by Mars. (Going to Future)
Mars has invaded Earth. Earth has been invaded by Mars. (Present Perfect)
Mars had invaded Earth. Earth had been invaded by Mars. (Past Perfect)
Mars will have invaded Earth. Earth will have been invaded by Mars. (Future Perfect)
I hate Mars invading Earth. I hate Earth being invaded by Mars. (Present Participle form)
Mars would invade Earth. Earth would be invaded by Mars. (Conditional Mood)
Mars must invade Earth. Earth must be invaded by Mars. (Modal Form; there are other modals besides must, including ought, can, should, may...)
Other kinds of structures are possible, but I don’t want to get too repetitive or technical.
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