A problem when changing the active voice into passive voice

[Dawood Usmani]

of course that i have already known some of the rules above.But what i asked you mean that : in the 2nd sentence,usually the criminals is arrested by the police,so you have known that the Subject (the police)is clear,why don't you remove it from the Passive sentence???

When the agent ( doer) is clear, you don't need to mention it. You could remove it. Is that what you want to know? Does that help?
Dawood

[RonBee]

of course that i have already known some of the rules above.

Of course, I already know some of the rules.

usually the criminals is arrested by the police

usually the criminals are arrested by the police

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The passive voice can be quite useful. For example, if you don't want to sound accusatory you might use the passive voice. Example:

Active voice:

Mr. X sent me to the wrong place.

Passive voice:

I was sent to the wrong place.