In the following excerpt from the 18th-century United States Declaration of Independence (1776), the bold text identifies passive verbs; italicized text identifies the one active verb (hold ) and the copulative verb are:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
In this case, the agent ("the Creator") of the passive construction can be identified with a by phrase. When such a phrase is missing, the construction is an agentless passive. For example, "Caesar was stabbed" is a perfectly grammatical full sentence, in a way that "stabbed Caesar" and "Brutus stabbed" are not. Agentless passives are common in scientific writing, where the agent may be irrelevant (e.g. "The mixture was heated to 300Â°C").
Passive constructions serve a number of useful purposes. First, you can change the emphasis of a sentence by moving the goal to the subject of the sentence. For example, if the topic of a paragraph is a bridge rather than the engineers who built it, you can make the bridge the subject of the sentence by intentionally using a passive construction: The Golden Gate bridge was designed to combine form and function to dramatic effect. Because agents can be omitted, passive constructions are particularly useful when the agent is unknown (The program was written in C++) or when the agent is obvious or unnecessary (The project was finally completed last night).
Because the agent can be omitted, the passive voice also allows you to avoid appearing to blame someone. That is, you can say that a device was damaged without mentioning who damaged it. But you can also use passive constructions to avoid responsibility. That is, you can say that a decision was made without admitting that you made the decision. Unfortunately, if you omit agents without thinking, your readers may assume that you are avoiding responsibility when that is not your intention.
One of the reasons passive sentences are so common in technical and scientific writing is that they allow you to communicate an impression of objectivity. After the experiment was completed, the data was analyzed sounds more objective than After I completed the experiment, I analyzed the data. The notion that scientific objectivity should be reflected in scientific writing may well explain why some writers avoid using active voice and first-person pronouns.
You might also want to question the value of this objectivity in some situations. For example, compare the procedure was changed (a passive construction) with we changed the procedure (its active counterpart). The procedure did not change spontaneously; someone changed the procedure. Who did it may be useful information. Omitting the agent may leave readers wondering who made the change, distracting their attention by raising a question and thus disrupting the flow of information.
Another reason to pay attention to passive constructions is that they can be less clear than active ones. In fact, many writers unconsciously shift to passive sentences when they are unsure of what they are saying or are struggling to express their thoughts. In some cases, if you take a passive sentence and rewrite it as an active one, you will realize that the passive sentence did not express your ideas correctly. If the active sentence is more clear for you as the writer, it will certainly be more clear for readers.