Active Voice and Passive Voice

To change a sentence from active voice to passive voice, make the object the new subject. In active voice, the subject of the verb is doing the action, but in passive voice the subject of the verb is being acted on. To change a sentence to passive voice, make the object the subject and use the correct form of the verb 'be' with the past participle of the main verb. Then the original subject can be used in a prepositional phrase beginning with 'by.'

Mary ate the food. → The food was eaten by Mary.

In this example, we use 'was' in the passive sentence because in the active sentence the main verb 'ate' is past tense. 'Eaten' is the past participle of the main verb 'eat.' 'Mary,' the person doing the action, is in the prepositional phrase "by Mary." Below are two more examples with the verbs in bold.

My father bought a red car. → A red car was bought by my father.
Babies eat oatmeal. → Oatmeal is eaten by babies.

In the following examples each active sentence has an auxiliary verb (be, have) so each passive sentence has three verb elements instead of two. The correct form of the helping verb is first, then the correct form of the verb 'be,' and then the past participle of the main verb. The verbs are in bold.

I am sweeping the rooms. → The rooms are being swept by me.
Francis has eaten the cake. → The cake has been eaten by Francis.

Read more about the passive voice here. 

I hope this helps.