Passive voice is a grammatical voice common in many of the world’s languages. In a clause with passive voice, the grammatical subject expresses the theme or patient of the main verb – that is, the person or thing that undergoes the action or has its state changed. This contrasts with active voice, in which the subject has the agent role. For example, in the passive sentence “The tree was pulled down”, the subject (the tree) denotes the patient rather than the agent of the action.
Plain English guidelines advice against passive voice in many document types. If you are unsure about using passive or active voice, ask yourself this question. Do I know the subject (the agent)? If you know it, use active voice.
The second question to ask is the importance of object. If the object is more important, passive voice typically suits better.
Example – active: You need a visa to enter the EU.
Example – passive: The glass is broken.