Writing teachers often tell their students not to use passive verbs. What they really mean is this: you should make sure that the subjects of your sentences are also the main characters of the story you’re telling.
Passive verbs are only a problem if they hide the main characters of your story. For example, if you write the sentence “Gaul was conquered” or “The idea was approved,” most readers will ask: by whom? But the root problem with those constructions isn’t that they’re passive; it’s that they hide important characters.
So, passive verbs aren’t always bad. If a passive verb lets you foreground the sentence’s most important character, it can actually be clearer than an active verb. You might decide to use passive verbs in two situations:
When you want to keep the focus of your story on a single main character, passive voice can let you keep that character in the topic position of your sentences.
Until the Gallic Wars of 58-51 BC, Gaul could not produce enough food to support its citizens, primarily because irrigation systems were unknown in the region. Once Gaul was subdued by Caesar, however, it was taught by its conquerors the Roman techniques for crop irrigation.
These sentences tell a story about one main character: Gaul. This writer puts it in the topic position of all of her sentences because she wants to keep the reader focused on it, and not any of the other characters. (Notice that, even so, she anticipates the reader’s question “who conquered Gaul?”)
When the agent of an action is unimportant to the story you want to tell, passive voice can let you remove that character from the topic position.
Prior to the optimization runs, the steam efficiency of the double-effect evaporator’s second effect was measured to be 61-63%.
Who measured the steam efficiency? The writer’s team of engineers. The author could have written “we measured the steam economy,” but most scientific writing doesn't mention who does the measuring, since valid empirical results do not depend on that. Since the people doing the measuring aren’t relevant to the story, the writer wisely removes them from the sentence and uses a passive-voice construction to put the important character (“steam economy”) in the topic position.
The Bottom Line: So long as they do not obscure your story’s important characters, passive verbs aren’t bad. Sometimes they can even help you emphasize the characters you want your readers to notice.