So when is it OK to use passive voice?

Passive voice is useful and necessary when you don't know who is acting, when what was done is more important than who did it, and when you want to speed up the narrative.

When you don't know who is acting:

A fern was crushed. The tracker stooped down to inspect it.

Who crushed the fern? The tracker doesn’t know because he wasn't there when it happened, so he can't tell us.

When what was done is more important than who did it:

The Gingerbread Man was killed today,
He'll be desperately missed.
Never again will he run away,
Or be hugged, or petted, or kissed.

What is important is that the beloved Gingerbread Man has died. It doesn't matter how.

Later. . .later dazed people will be able to think again. Then they will ask, "Who killed the Gingerbread Man?" But at first all they can take in is that their Gingerbread Man is dead.

If we give too many details right away, those details will be lost, because at the time of great emotion, readers can't take everything in. So, if you have a traumatic event in you novel, try writing it in passive voice at first and then feeding in the details about who did what later. And see if the passive accounting doesn't actually make the whole event more powerful than an active reporting of the event.

When it doesn't matter who was acting and you want to speed the story so the readers feel like they are making progress:

Usually passive writing slows the pace. But sometimes, I think, it speeds the pace and adds variety to scenes that would otherwise plod along.

Half an hour later the winter boots and backpacks were tied to the corners of the tiny raft with rope Mr. Habakkuk had grudgingly provided. Nate was tethered to one backpack, Connie to the other, and the two parkas were spread across the top of the raft making a soft seat for Habakkuk.

That whole paragraph is a passive summary of what happened in a half hour. To tell it actively wouldn't take more than a paragraph, so why use the passive voice? Because an active paragraph here actually slows the story down.

Mr. Habbakkuk conjured up rope for the kids to use. Nate tied the winter boots and the two backpacks to the corners of the tiny raft and then he tethered himself to the one backpack and Connie to the other. After that, he and Connie spread their parkas across the top of the raft, making a soft seat for Mr. Habakkuk.

The first paragraph is 53 words and the second 61. So why do I think the passive is better in this instance? Because in the first paragraph the reader sees that a half an hour has passed and junk has been accomplished. It gives the feeling of speed. The second paragraph slows things down as we watch an actor perform each uninteresting task. And since there is no reason the reader needs to know who does what here, it's fine to leave the actor out of the picture.

So go ahead and try the passive voice sometimes.

Sparingly.

It may serve as a nice change-up for your manuscript.