In English grammar, the "voice" describes the relationship between the verb and its participants. If the subject of the sentence does the action, the verb is in the active voice. If the subject does not actively participate in the action described and the focus is on the action itself, not the subject, then the verb is in the passive voice.

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It is important when you write in English that you distinguish between the active and passive voice. If you want the focus to be on the subject, or actor, use the active voice. If you wish the emphasis to be on the action itself, and not the actor, use the passive voice. 

Here are two examples of the active voice from Yabla English: 
 

He created the mythology.

Caption 54, New Zealand 100% Pure - 'The Hobbit' Cast Talks about New Zealand

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Now, we'll have a demonstration from some of your instructors.

Caption 15, Karate Kids, USA - The Little Dragons

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In the first example, the focus is on the subject "he" having created the mythology. It is usually easy to make a passive voice sentence out of the active by using the verb "to be" and the past participle of the original verb. In this case, we can write it in passive voice thus: 

The mythology was created by him. 

The mythology did not do the creating. The focus here is not the fact that he created it, but the fact that it was created. 

In the second example, we can render it passive like this: 

Now, there'll be a demonstration for us from some of your instructors. 

"We" are no longer emphasized as the ones who will be the audience of the demonstration, but rather the fact of the demonstration is the most important thing.

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Further Learning
Read more about the active voice and the passive voice and find examples on Yabla English to see them used in a real-world context.