Changing active to passive voice formula and example?

You can change the active voice to the passive voice by changing an object to a subject, as follows: let us say that your original sentence is Fred has eaten dinner. That is the active voice. Dinner is the object. In the passive voice dinner becomes the subject, so the sentence becomes, dinner has been eaten. Fred, the original subject, has disappeared. We no longer know who ate the dinner, we merely know that it has been eaten by someone. The passive voice is therefore distinguished by being less informative. It is used by people who are trying to evade responsibility for something, or to avoid being specific. Something happened but we can't say who did it.

The form of passive is be + past participle

eg - been eaten, being eaten, was cooked, is cooked,

The passive allows us to leave the 'doer' of the verb out of the sentence. This is useful if the 'doer' is not important. eg

The Mona Lisa was painted in the 16th century.

If you want to say who does the action of a passive verb add by + pronoun / noun phrase at the end of the sentence. This is called the agent.

The dinner was eaten by Fred.

This is useful if you want to emphasis the agent eg

The painting is very valuable. It was painted by Van Gogh.