The passive voice is the contrary of the active voice. With the active voice the subject is doing the action. With the passive voice the subject is not doing the action but is experiencing the action done by someone else. This action is done by the object also called the agent and can be removed.
Active voice → the cat eats the mouse
Passive voice → the mouse is eaten (by the cat)
therefore, the object of the active voice becomes the subject in the passive voice. For the verb with the passive voice the auxiliary « be » is added using the same tense than the active voice followed by the past participle.
Present | The cat eats the mouse → the mouse is eaten (by the cat)
Present continuous | The cat is eating the mouse → the mouse is being eaten (by the cat)
Preterit | The cat ate the mouse → the mouse was eaten (by the cat)
Present perfect | The has eaten mice → the mice have been eaten (by the cat)
⚠️Always agree the subject with the auxiliary
the mouse is / was / has been & the birds are / were / have been