[matilda]

you know, passive voice doesn't sound like perfect form AT ALL.
hey are different in use, and in usage. as you use passive, you mean you dont know the doer of the action. for example:
Alex is killed.
that means that someone who we don't know has killed alex in a past time before now, and the act of kiling is finished now.
but sometimes, we KNOW the does of the action, but the action is more important than the doer.look at the following example:
Alex was likked by Marry.
in this sentence, we know that Marry has killed ales before, but the action of killing is much more impotrtant than who the murderer is.

Hope that helps

Matilda