At one time or another, all writers have probably seen someone warning them not to use forms of the verb "to be" because that would be "passive voice" and that's "bad writing." The trouble is that frequently neither the giver nor the receiver of that advice understands what's being talked about. To understand, you have to differentiate between passive voice, emphasis on action, and the delights of characters who shape situations rather than just respond to them.
The terms "active voice" and "passive voice" apply only to transitive verbs, i.e. verbs that pass action from an actor to a receiver. In an active voice sentence, the subject of the sentence is the actor.
Active voice example: John hit the ball.
In that sentence, "John" is both the grammatical subject of the sentence and the one who's doing the hitting. In contrast, in a passive voice sentence, the subject of the sentence is the receiver of the action.
Passive voice example: The ball was hit by John.
In that sentence, "ball" is the subject and it's being hit.
It's important to realize that the passive voice doesn’t always constitute bad writing. Passive voice is especially useful in two situations. (1) If you want to hide the actor, you can omit the "by" phrase and get "The ball was hit" with no indication of who did it. (2) If you want to emphasize the receiver of the action because the actor is unimportant. If you're writing about the history of a building, for instance, you might not care who the builder was so you could write "Ross Hall was built in 1956."
However, generally speaking, it's a good idea to edit out passive voice sentences because they're measurably slower to read and harder to comprehend. Readers want to know who's kicking whom, and that means active voice.
As you see, though, it isn't the use of "to be" that's a problem in the passive voice. The verb "to be" is not a transitive verb because it doesn't transmit action, so by itself, it's neither active nor passive. You see it as an auxiliary to "hit" to form the passive voice in "The ball was hit by John," but as I say, by itself, no form of "to be" is active or passive. So you can't rule out using it by calling it "passive voice."
Emphasis on Action
Instead, "to be" conveys a state of being. "He is tall." "The room was dark." Overuse of "to be" creates a problem not because it's passive voice but because there's no action, not even action passively expressed. The story isn't moving along. The situation is static. That's why advice about writing description often says to do it in action. Rather than saying, "The room was dark," have your character act in the dark room and thereby show it to be dark. For instance, you could say, "He put out his hands to keep from tripping in the coal dark."
Active vs passive characters
Another sort of "passive" has to do with passive characters. Once again, this has nothing to do with passive voice. Rather, it refers to characters, especially MCs, who react to events rather than shaping them. By and large, we cheer for active characters more than passive ones, who can be seen as victims. We admire characters who persist and get up to try again after they're knocked down. We like them to have a goal they're struggling to reach, so we can hope along with them.
Mind you, as with everything else, that's not absolute. You can undoubtedly think of passive characters whose story you enjoyed. But usually, we like active characters.
So passive voice, use of "to be," and passive characters are three different things that get lumped together when well-meaning but wrong advisers tell someone to avoid the verb "to be" at all costs.
Dorothy Winsor lives in Iowa and writes young adult fantasy.