When a sentence is in passive voice, the subject is being acted on by the verb and the subject typically comes after the action. For example:
Swimming is something I do.
Here, the action is swimming. The subject is I. The sentence is in passive voice since the person doing the action (I) is not mentioned until after the action.
Some sentences also contain objects - the thing being acted upon. This can make it more difficult to determine whether the sentence is in passive voice. For example, here is a sentence in active voice:
Anna hits the ball.
Anna is the subject. Hits is the action. The ball is the object.
That same sentence in passive voice reads:
The ball is hit by Anna.
The ball is the object, not the subject of the sentence. The ball is not doing an action. Therefore, it should be after the subject (Anna).
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