We were given new toothbrushes by the hygienist.
Passive voice happens when the subject of the sentence is not the one actually doing the action of the main verb. In this sentence the verb phrase is "were given". The main verb is "given". "We" are not the ones who did the giving. The hygienist is the one giving out new toothbrushes, but it's the object of the preposition not the subject of the sentence. To correct the passive voice, the sentence would need to be rewritten to: The hygienist gave us new toothbrushes.
