The primary reason why your instructors frown on the passive voice is that they often have to guess what you mean. Sometimes, the confusion is minor. Let’s look again at that sentence from a student’s paper on Homer’s The Odyssey:
When her house was invaded, Penelope had to think of ways to delay her remarriage.
Like many passive constructions, this sentence lacks explicit reference to the actor—it doesn’t tell the reader who or what invaded Penelope’s house. The active voice clarifies things:
After suitors invaded Penelope’s house, she had to think of ways to fend them off.
Thus many instructors—the readers making sense of your writing—prefer that you use the active voice. They want you to specify who or what is doing the action. Compare the following two examples from an anthropology paper on a Laotian village to see if you agree.
(passive) A new system of drug control laws was set up. (By whom?)
(active) The Lao People’s Revolutionary Party set up a new system of drug control laws.
Here’s another example, from the same paper, that illustrates the lack of precision that can accompany the passive voice:
Gender training was conducted in six villages, thus affecting social relationships.
And a few pages later:
Plus, marketing links were being established.
In both paragraphs, the writer never specifies the actors for those two actions (Who did the gender training? Who established marketing links?). Thus the reader has trouble appreciating the dynamics of these social interactions, which depend upon the actors conducting and establishing these things.
The following example, once again from that paper on The Odyssey, typifies another instance where an instructor might desire more precision and clarity:
Although Penelope shares heroic characteristics with her husband, Odysseus, she is not considered a hero.
Who does not consider Penelope a hero? It’s difficult to tell, but the rest of that paragraph suggests that the student does not consider Penelope a hero (the topic of the paper). The reader might also conceivably think that the student is referring to critics, scholars, or modern readers of The Odyssey. One might argue that the meaning comes through here—the problem is merely stylistic. Yet style affects how your reader understands your argument and content. Awkward or unclear style prevents your reader from appreciating the ideas that are so clear to you when you write. Thus knowing how your reader might react enables you to make more effective choices when you revise. So after you identify instances of the passive, you should consider whether your use of the passive inhibits clear understanding of what you mean.