Teachers actually get fired up about this issue. You may have had a frustrated (and frustrating?) professor write on your paper "Use passive voice!" or "Avoid passive voice!" during your studies, and your grammar checker will be happy to flag and condemn all passive sentences for you. Further, your English textbook might suggest that the active sentence "Jack hit the baseball" is better than the passive sentence "The baseball was hit by Jack." As well-intentioned as they might be, these tidbits of advice don’t help much, do they? You are not likely to have anyone named Jack hitting any baseballs in your papers, and obviously both passive and active voice are powerful tools in the right hands.
You are probably already able to identify whether or not sentences are written in the passive or active voice, but if not, here is a refresher: In the simplest terms, an active voice sentence is written in the form of "A does B." A passive voice sentence is written in the form of "A is done by B." Both constructions are fine. In fact, notice how the sentences below, depending on the context in which they appear, are equally acceptable:
Passive voice: The rate of evaporation is controlled by the size of an opening.
Active voice: The size of an opening controls the rate of evaporation.
The passive choice slightly emphasizes "the rate of evaporation," while the active choice emphasizes "the size of an opening." Simple. So why all the fuss? Because the habit of overusing passive constructions rules too many writers, who habitually produce grammatically tangled sentences such as this one:
Groundwater flow is influenced by zones of fracture concentration, as can be recognized by the two model simulations (see Figures 1 and 2), by which one can see . . .
Forget it. The sentence is becoming a burden for the reader, and probably for the writer too. As often happens, the passive voice here has smothered potential verbs and kicked off a runaway train of prepositions. But the reader’s task gets much easier in the revised version below:
Two model simulations (Figures 1 and 2) illustrate how zones of fracture concentration influence groundwater flow. These simulations show . . .
To revise the above, all I did was look for the two buried things (simulations and zones) in the original version that could actually do something, and I made the sentence clearly about these two nouns by placing them in front of active verbs. This is the general principle to follow as you compose in the active voice: Place concrete nouns that can perform work in front of active verbs representing the nature of the work done.
But suppose you are writing a report where you may not use "I", or you are writing about a sentence subject that can not actually do anything. What to do when the passive voice is the best, most natural choice?
The answer lies in writing direct sentences—in passive voice—that have simple subjects and verbs. Compare the two sentences below:
Photomicrographs were taken to facilitate easy comparison of the samples.
Easy comparison of the samples was facilitated by the taking of photomicrographs.
Both sentences are written in the passive voice, but for most ears the first sentence is more direct and understandable, and therefore preferable. Depending on the context, it does a clearer job of telling us what was done and why it was done. Especially if this sentence appears in the "Experimental" section of a report (and thus readers already know that the authors of the report took the photomicrographs), the first sentence neatly represents what the authors actually did—took photomicrographs—and why they did it—to facilitate easy comparison.
Note well: Using passive voice does not have to create ambiguity nor complicate wording. When you use the passive voice, seek economy and clarity. Avoid such empty and ambiguous phrases as "it might be thought that" (try "perhaps") or "it is to be supposed that" (try "presumably") or "the theory that is held by the writer of this report at the present time of this writing" (try "It is theorized that") or "one should think of" (try dropping it completely). At times the passive seems unavoidable, but the passive can often be switched to the active with some simple rewording, and both the active and the passive voice can be direct, efficient, and clear in context. In your writing, you must strive to use both of them well.
For examples and exercises on passive vs. active voice, check out these websites: