How to change this to active voice?

The matter was reported to the commissioner.

My teacher told me that it should be

The commissioner reported the matter.

But I think that this is not correct since it implies the commissioner was the one who reported the matter, which is incorrect.

My guess is

Someone reported the matter to the commissioner

Is this correct?

The matter was reported to the commissioner.

and

The commissioner reported the matter.

mean different things. To change to the active voice while retaining the meaning would be something like this:

The commisioner received the report of the matter.

This is a more verbose way to say it, and it is a more complicated and less natural construction. I woud recommend using the original passive example. The passive voice is just as grammatically correct as the active, and in this case it better suits the semantics of the sentence.

If you are able to specify a subject of the sentence, such as "he" or "they" or "the deputy", then you could reconstruct the sentence like this:

He/they/the deputy reported the matter to the commissioner.

This doesn't retain the original meaning because it introduces a subject that was never mentioned in the original sentence, but it may suit your purposes well enough.

You're right when you say that

The commissioner reported the matter.

Implies the commissioner was the one reporting the matter. It is only correct if the objective was to use the active voice.

The active voice requires that the subject of the sentence is performing some action.

Your original sentence,

The matter was reported to the commissioner

has "The matter" (subject) being reported by someone else. Because "The matter" is an inanimate object, it cannot report itself, so you need a subject that CAN perform some action!

Thus, you may want to change the subject of the sentence to something else while still retaining its meaning. Something like:

The secretary reported the matter to the commissioner

You are right if the objective was to change the sentence to active while leaving its meaning intact: if that was the objective, there is no satisfactory solution. I suppose you could say "they reported the matter to the commissioner" or something like that.

Looking for an actor is one of the easiest ways to spot active/passive voice, and one of the reasons why passive voice is used so often. The only way to make this sentence active is to work out who the actor is, even if it is just "they" (which could be plural or singular), like Cerberus mentioned.

L