Passive voice in a sentence is characterized by a focus on the action being done to the subject, rather than who or what is performing the action. In a passive construction, the subject receives the action rather than performing it.
For example, in the sentence "The book was read by the student," "the book" is the subject that is receiving the action of being read, while the doer of the action (the student) is mentioned after the verb and usually preceded by "by."
In contrast, an active voice sentence would be "The student read the book," where the subject (the student) performs the action.
In summary, passive voice typically follows this structure: Subject + form of "to be" + past participle + (optional agent).