Passive grammar refers to one of two possible ways to express action within a sentence or clause. In the active voice the subject acts upon an object or conducts the action. The passive voice changes the order of the sentence or clause so that the object, or recipient, of the action is the subject of the sentence. Using passive grammar is not a grammatical mistake but a stylistic preference, which many believe weakens or distorts the concept or idea that the sentence attempts to convey.
In order to understand the difference between passive and active voice, one must understand the elements of a sentence or clause, a grammatical unit containing a complete idea. The agent is the grammatical component that causes the action to take place, the thing or person that is doing something. Most of the time, this component is in the subject position, causing the action.
The verb is the part of the sentence that expresses action, whether upon another thing or within itself, "He shoots the ball," or "He shoots," respectively. The object is the grammatical element that receives the action, in this case, "the ball." The agent acts upon the object by means of the verb's action.
The most common formation of the passive voice combines a form of the verb "to be," with the past participle of the main verb. The example, "Peter drove the car." is a sentence written in the active voice, as Peter, the agent, is in the subject position acting upon the car, the object. To write the sentence in the passive voice, one would rearrange the sentence to read, "The car was driven by Peter." Here the car, the recipient of the action, is the subject, and it is followed by "was," and the past participle of the verb "to drive," which is "driven." Only after providing this information does the writer tell the reader that Peter is the force acting upon the car.
In the passive voice sentence, "The car was driven by Peter." the car is the object of the action, while Peter is the agent causing the action to take place. Use of the passive voice suggests that the writer intends to place more focus on the car than the driver. In the active voice, the sentence becomes, "Peter drove the car," where the the subject is the agent, the one doing the action. Unless the car is actually more important than Peter, most writers would agree that the active version is preferable, as it is more emphatic and more straightforward, leaving less room for reader error or misunderstanding.
If the agent is removed from the sentence, it becomes, "The car was driven." This sentence illustrates another usage of the passive voice, in which the object of the action and the action itself are the only things that the writer intends the reader to know. Here the writer uses passive grammar to keep the agent ambiguous, implying that the agent is unimportant or unknown, possibly even to the writer.
Just because a sentence contains a form of the verb to be does not mean that the sentence or clause uses passive grammar. The same is true for sentences using a past participle. The verb "to be" can also denote a state of being, such as in the sentences, "John is sick." and "John was sick." In these cases the "is" and "was" are used to describe John's physical state or condition, with the action existing as a continuation from one moment to the next in present and past time, respectively.