My students have been taught that a verb is in the passive voice whenever a form of the verb to be appears. They have also been taught that it is never to be used. They are wrong on both counts.
I’ll speak about the use of the passive voice later. For now, let’s define what we mean by voice. Consider these three sentences:
Superman was stopped by Lex Luthor and a very large dose of kryptonite.
Superman stopped the train with one hand tied behind his back.
“I wish you wouldn’t fly away so fast!” said Lois. Superman stopped.
Voice denotes the relationship between the subject and the verb. Is the subject performing the action of the verb? Then that verb is in the active voice. Is the subject “performing” the state of being named by the verb? For example: “Superman is a fink.” Superman is performing the being-a-fink. That verb is is in the active voice. But if the subject is the sufferer of the action named by the verb, as in the first sentence above—the kryptonic Lex is stopping Superman, not the other way around—then the verb is in the passive voice, literally the suffering-the-action voice. In English, we form the passive voice by using a form of the verb be, followed by the past participle, but that’s just what we happen to do. We use forms of be all the time, without forming the passive voice. “Superman is a fink.” “Superman is picking that old man’s pocket.” “Superman has never been here.” All those are in the active voice. Other languages, like Latin and Greek, form the passive voice without the verb be at all. The one doesn’t necessarily have anything to do with the other.
The third example above is considered active voice in English, but in Greek it would be cast in the middle voice, between active and passive. Superman is doing the stopping, sure; but he is also suffering the stopping. He is stopping himself. The Greeks “heard” a difference there, and employed a different form of the verb. So do speakers of many other languages. It’s a nice tool to have, that middle voice.