What is passive voice? assive: The uncertainty principle was formulated by Werner Heisenberg in 1927. In an active sentence, the person or thing responsible for the action in the sentence comes first. In a passive sentence, the person or thing acted on comes first, and the actor is added at the end, introduced with the preposition "by." The passive form of the verb is signaled by a form of "to be": in the sentence above, "was formulated" is in passive voice while "formulated" is in active. In a passive sentence, we often omit the actor completely: The uncertainty principle was formulated in 1927. When do I use passive voice? In some sentences, passive voice can be perfectly acceptable. You might use it in the following cases: The actor is unknown: The cave paintings of Lascaux were made in the Upper Old Stone Age. [We don't know who made them.] The actor is irrelevant: An experimental solar power plant will be built in the Australian desert. [We are not interested in who is building it.] You want to be vague about who is responsible: Mistakes were made. [Common in bureaucratic writing!] You are talking about a general truth: Rules are made to be broken. [By whomever, whenever.] You want to emphasize the person or thing acted on. For example, it may be your main topic: Insulin was first discovered in 1921 by researchers at the University of Toronto. It is still the only treatment available for diabetes. You are writing in