Passives are grammatical constructions where the subject of a sentence receives the action rather than performing it. In a passive sentence, the focus is on the action and the recipient, rather than the doer. For example, in the sentence "The book was read by John," the emphasis is on the book, not on John. In English, passive voice is formed using a form of the verb "to be" followed by the past participle of the main verb. This structure can be useful for emphasizing the action or when the doer is unknown or irrelevant. For instance, "The cake was eaten" highlights the cake rather than who ate it.